How is rearranging $56times 100div 8$ into $56div 8times 100$ allowed by the commutative property?
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So according to the commutative property for multiplication:
$a times b = b times a$
However this does not hold for division
$a div b neq b div a$
Why is it that in the following case:
$56 times 100 div 8 = 56 div 8times 100$
It seems like division is breaking the rule. There is something I am misunderstanding here.
Is it because $atimes bdiv c=adiv ctimes b$ is allowed since $bdiv c$ are not being rearranged so that $cdiv b$?
If this is the case are you allowed to rearrange values in equations so long as no values have the form $a div b = b div a$ and $a - b = b -a$ ?
arithmetic
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add a comment |
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So according to the commutative property for multiplication:
$a times b = b times a$
However this does not hold for division
$a div b neq b div a$
Why is it that in the following case:
$56 times 100 div 8 = 56 div 8times 100$
It seems like division is breaking the rule. There is something I am misunderstanding here.
Is it because $atimes bdiv c=adiv ctimes b$ is allowed since $bdiv c$ are not being rearranged so that $cdiv b$?
If this is the case are you allowed to rearrange values in equations so long as no values have the form $a div b = b div a$ and $a - b = b -a$ ?
arithmetic
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@marshalcraft: There is no reasonable sense in which multiplication and division are associative "together" -- as Mark Bennet's answer points out, $a/(bcdot c) ne (a/b)cdot c$ in general.
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– Henning Makholm
15 hours ago
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Indeed its an error on my part.
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– marshal craft
15 hours ago
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So I need to say is multiplication is commutative and associative, division is not associative and not commutative. But when you compute the result of division you remove the division and thus can then proceed to use multiplicative property of commutivity and associativity. K.
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– marshal craft
14 hours ago
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To would-be editors: please, please do not convert the $div$ operators in this question into the fractional form $frac ab.$ It changes the question completely.
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– David K
14 hours ago
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Also due to the non commutivity, and non associativity of division, the notation used above is decidably ambiguous. That means there are a number of equally valid ways to interpret expressions ( this is why different calculators can give different results). You can explore the possibilities. In higer level math fractions, parents, etc. are used and there is no ambiguity.
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– marshal craft
13 hours ago
add a comment |
$begingroup$
So according to the commutative property for multiplication:
$a times b = b times a$
However this does not hold for division
$a div b neq b div a$
Why is it that in the following case:
$56 times 100 div 8 = 56 div 8times 100$
It seems like division is breaking the rule. There is something I am misunderstanding here.
Is it because $atimes bdiv c=adiv ctimes b$ is allowed since $bdiv c$ are not being rearranged so that $cdiv b$?
If this is the case are you allowed to rearrange values in equations so long as no values have the form $a div b = b div a$ and $a - b = b -a$ ?
arithmetic
$endgroup$
So according to the commutative property for multiplication:
$a times b = b times a$
However this does not hold for division
$a div b neq b div a$
Why is it that in the following case:
$56 times 100 div 8 = 56 div 8times 100$
It seems like division is breaking the rule. There is something I am misunderstanding here.
Is it because $atimes bdiv c=adiv ctimes b$ is allowed since $bdiv c$ are not being rearranged so that $cdiv b$?
If this is the case are you allowed to rearrange values in equations so long as no values have the form $a div b = b div a$ and $a - b = b -a$ ?
arithmetic
arithmetic
edited 13 hours ago
Asaf Karagila♦
305k32435765
305k32435765
asked 22 hours ago
SphygmomanometerSphygmomanometer
919
919
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@marshalcraft: There is no reasonable sense in which multiplication and division are associative "together" -- as Mark Bennet's answer points out, $a/(bcdot c) ne (a/b)cdot c$ in general.
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– Henning Makholm
15 hours ago
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Indeed its an error on my part.
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– marshal craft
15 hours ago
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So I need to say is multiplication is commutative and associative, division is not associative and not commutative. But when you compute the result of division you remove the division and thus can then proceed to use multiplicative property of commutivity and associativity. K.
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– marshal craft
14 hours ago
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To would-be editors: please, please do not convert the $div$ operators in this question into the fractional form $frac ab.$ It changes the question completely.
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– David K
14 hours ago
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Also due to the non commutivity, and non associativity of division, the notation used above is decidably ambiguous. That means there are a number of equally valid ways to interpret expressions ( this is why different calculators can give different results). You can explore the possibilities. In higer level math fractions, parents, etc. are used and there is no ambiguity.
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– marshal craft
13 hours ago
add a comment |
$begingroup$
@marshalcraft: There is no reasonable sense in which multiplication and division are associative "together" -- as Mark Bennet's answer points out, $a/(bcdot c) ne (a/b)cdot c$ in general.
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– Henning Makholm
15 hours ago
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Indeed its an error on my part.
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– marshal craft
15 hours ago
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So I need to say is multiplication is commutative and associative, division is not associative and not commutative. But when you compute the result of division you remove the division and thus can then proceed to use multiplicative property of commutivity and associativity. K.
$endgroup$
– marshal craft
14 hours ago
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To would-be editors: please, please do not convert the $div$ operators in this question into the fractional form $frac ab.$ It changes the question completely.
$endgroup$
– David K
14 hours ago
$begingroup$
Also due to the non commutivity, and non associativity of division, the notation used above is decidably ambiguous. That means there are a number of equally valid ways to interpret expressions ( this is why different calculators can give different results). You can explore the possibilities. In higer level math fractions, parents, etc. are used and there is no ambiguity.
$endgroup$
– marshal craft
13 hours ago
$begingroup$
@marshalcraft: There is no reasonable sense in which multiplication and division are associative "together" -- as Mark Bennet's answer points out, $a/(bcdot c) ne (a/b)cdot c$ in general.
$endgroup$
– Henning Makholm
15 hours ago
$begingroup$
@marshalcraft: There is no reasonable sense in which multiplication and division are associative "together" -- as Mark Bennet's answer points out, $a/(bcdot c) ne (a/b)cdot c$ in general.
$endgroup$
– Henning Makholm
15 hours ago
$begingroup$
Indeed its an error on my part.
$endgroup$
– marshal craft
15 hours ago
$begingroup$
Indeed its an error on my part.
$endgroup$
– marshal craft
15 hours ago
$begingroup$
So I need to say is multiplication is commutative and associative, division is not associative and not commutative. But when you compute the result of division you remove the division and thus can then proceed to use multiplicative property of commutivity and associativity. K.
$endgroup$
– marshal craft
14 hours ago
$begingroup$
So I need to say is multiplication is commutative and associative, division is not associative and not commutative. But when you compute the result of division you remove the division and thus can then proceed to use multiplicative property of commutivity and associativity. K.
$endgroup$
– marshal craft
14 hours ago
$begingroup$
To would-be editors: please, please do not convert the $div$ operators in this question into the fractional form $frac ab.$ It changes the question completely.
$endgroup$
– David K
14 hours ago
$begingroup$
To would-be editors: please, please do not convert the $div$ operators in this question into the fractional form $frac ab.$ It changes the question completely.
$endgroup$
– David K
14 hours ago
$begingroup$
Also due to the non commutivity, and non associativity of division, the notation used above is decidably ambiguous. That means there are a number of equally valid ways to interpret expressions ( this is why different calculators can give different results). You can explore the possibilities. In higer level math fractions, parents, etc. are used and there is no ambiguity.
$endgroup$
– marshal craft
13 hours ago
$begingroup$
Also due to the non commutivity, and non associativity of division, the notation used above is decidably ambiguous. That means there are a number of equally valid ways to interpret expressions ( this is why different calculators can give different results). You can explore the possibilities. In higer level math fractions, parents, etc. are used and there is no ambiguity.
$endgroup$
– marshal craft
13 hours ago
add a comment |
9 Answers
9
active
oldest
votes
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Notice that you have always $div 8$, no matter the order of the other terms. You don't divide by a different number. It might help to think $div c$ as a multiplication with $d=1/c$. Then everything would look easier:
$$atimes bdiv c=atimes btimes d=atimes dtimes b=adiv ctimes b$$
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add a comment |
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Hope this makes sense.
$$ atimes b ÷ c $$
$$=atimesdfrac{b}{c}$$
$$=atimes btimesdfrac{1}{c}$$
$$=(atimesdfrac{1}{c})times b$$
$$=dfrac{a}{c}times b$$
$$=a÷ctimes b$$
New contributor
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The first step is not elementary. The literal interpretation of the order of operations in $atimes b div c$ is $(atimes b) div c$. what gives us the ability to take the $b$ out of the first operation and put it into $frac bc$? I think this answer would be better if it went straight from $atimes b div c$ to $atimes b times frac1c$.
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– David K
14 hours ago
add a comment |
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Because division is the inverse of multiplication, that is: $$X div Y =Xcdot frac 1Y$$
So you have: $$56cdot 100 cdot frac18 =56cdotfrac18cdot100$$
Which is obvious.
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While it is true that "division is the inverse of multiplication" the emphasis should be on what follows from it: That division is equivalent to multiplication with the inverse and can be replaced by it.
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– Peter A. Schneider
20 hours ago
add a comment |
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Others have answered the direct question, in that multiplication is commutative and that applies also to multiplication by a reciprocal (the equivalent of division). However there is an issue here with associativity and division which I think is worth mentioning. This has to do with the order in which operations are carried out.
So $adiv b times c$ is being interpreted from left to right as $(adiv b)times c=cfrac {ac}b$, but done from right to left $adiv (btimes c)=cfrac a{bc}$ and the two results are not the same.
Likewise with $adiv b div c$ we have $(adiv b)div c=cfrac a{bc} neq adiv (bdiv c)=cfrac {ac}b$.
So the conventional assumption that multiplication and division are operations of equal status and are carried out from left to right does make a difference in these cases and changes the result. This may be what is feeding your intuition that there is a potential problem with order.
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add a comment |
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The non-zero real numbers form an abelian (commutative) group under multiplication. The notation $adiv b$ is shorthand for $ab^{-1}$.
So $ab^{-1}ne ba^{-1}$ but $56times100 times8^{-1}=56times 8^{-1}times 100$
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15
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(-1) this answer is almost certainly nearly useless; the set of people that would ask this question will be nearly disjoint from the set of people that understand this answer
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– DreamConspiracy
19 hours ago
1
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Additionally, it's not even clear he asking over reals, could be over natural numbers in which case there is no group with multiplication as group operation.
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– marshal craft
17 hours ago
add a comment |
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The exact same situation occurs with addition and subtraction instead of multiplication and division. Do you find it hard to justify rewriting say $a+b-c$ to $a-c+b$? And if not, which laws are you using to justify this? I would say associativity, commutativity, and the special equivalence $x-y=x+(-y)$ (which can be construed to be the definition of subtraction, although historically and in teaching subtraction is introduced before introducing negative numbers). In detail one has inserting parentheses to make explicit the one implied by convention)
$$eqalign{
a+b-c &= (a+b)-c \&= (a+b)+(-c) \&= a+(b+(-c)) \&
= a+((-c)+b) \&= (a+(-c))+b \&= (a-c)+b &= a-c+b}
$$
By perfect analogy, let us do that with '$times$' replacing '$+$', and '$div$' replacing '$-$':
$$eqalign{
atimes bdiv c &= (atimes b)div c \&= (atimes b)times (div c) \&= atimes (btimes (div c)) \&
= atimes ((div c)times b) \&= (atimes (div c))times b \&= (adiv c)times b &= adiv ctimes b}
$$
Note how I just invented the unary use of '$div$', where $div x$ of course means the multiplicative inverse of$~x$, just as $-x$ means its additive inverse. Now why did nobody think of that before? It could have avoided the ugliness of$~x^{-1}$, which has to borrow exponentiation and additive inverse to get multiplicative inverse.
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add a comment |
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Start with 56 × 100 ÷ 8.
Swap the first two factors which is definitely allowed: 100 × 56 ÷ 8.
Now put parentheses around part of it: 100 × (56 ÷ 8).
Again, swap: (56 ÷ 8) × 100.
Take off the parentheses: 56 ÷ 8 × 100.
And there you go! transforming one into the other using only multiplicative commutation (plus the fact that one of the factors commutated can be a product, not just a simple value).
New contributor
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2
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Now explain why putting parentheses around the last two terms in $100times56div8$ is OK but putting parentheses around the last two terms in $56div8times100$ is not OK. For that matter, why is it OK to swap the first two terms in $100times56div8$ but not the last two terms of $56div8times100$?
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– David K
14 hours ago
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The short answer to adding the parentheses is, "because it doesn't change the outcome". But you're right, a more rigorous explanation is needed, and I don't have one. To swap the last two terms of 56 ÷ 8 × 100 does require parentheses around them, and if they aren't already there, they can't be arbitrarily added without changing the whole thing.
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– Jennifer
14 hours ago
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A formal reason is that $56times100div8$ means $(56times100)div8$ by convention, and $(56times100)div8=(100times56)div8=(100times56)timesfrac18=100times(56timesfrac18)=100times(56div8).$ But that's making it more complicated than necessary. I think it's simpler if we start with $(56times100)div8=(56times100)timesfrac18$.
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– David K
13 hours ago
add a comment |
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Your confusion comes from a fact that is rarely spoken of in school.
The real numbers constitute a so called field where we have two operations, multiplication and addition. These two in their own create a so called group with the given set, that is the real numbers $mathbb{R}$.
This may sound really cryptic but let me clear it up. In a group you need a set $X$ and have one and only one operation $*$, you have a neutral element $e$ fullfilling that it doesnt change the element which is combined with it, all the members of the set can be combined using $*$ which will result in another element of the set $X$ and all the elements have so called inverses that is for each element $a$ in $X$ you can find another element $b$ such that $a*b=e$.
Now this is still a bit formal and cryptic so let us find an example. The whole numbers, also called as the integers $mathbb{Z}$ with usual addition forms a group.
You can add integers and you end up with integers, $0$ serves as the neutral element since any integer plus $0$ is the same integer, unchanged and you also have inverses the so called negative numbers $5 + (-5)=0$. Because of lazyness we usually write this as $5-5=0$ giving the false idea that we also have minus $"-"$ as an operation but that is WRONG.
Another exapmle could be the integers with the operation multiplication. We can multiply any integers together resulting another integer so it looks okay so far, we can find a neutral element, namely $1$ which fullfills the desired property, namely any integer times one is the same integer but we fail to find inverses. You cannot find an integer to multiply, say $5$ with to get the neutral element $1$. So this is NOT a group.
All possible fractions also callet as quotients $mathbb{Q}$ is a group with respect to multiplication (everything looks similar to what stands directly above here) but here you will find inverses. $5$ can be multiplied by $frac{1}{5}$ to get one. But once again this DOES NOT make division an operation. It is tempting to think of $frac{1}{5}$ as one divided by five which may suggest that division is an actual operation in this group but $frac{1}{5}$ is an element.
So after this short tour into abstract algebra let us consider your question again
Remember we are working in $mathbb{R}$, the field of real numbers which means that $mathbb{R}$ is a group with respect to both addition and multiplication. Or even better you could translate $frac{1}{8}$ as the inverse of $8$ with respect to multiplication
$$
frac{56}{8}cdot 100
$$
actually means that take three elements of $mathbb{R}$, namely $frac{1}{8}, 100$ and $56$ and multiply them together and since multiplication is commutative you may do this in any order. Try not to think of division as an operation rather than a way to express elements of this group.
BONUS FACT:
thinking this way also helps debunking mysteries about subtraction
$$
5-8neq 8-5
$$
which is inherently wrong since once again it suggests that minus is an operation. But using the correct way of thinking, that is the minus sign is "glued" to a number as an attribute so to say you get
$$
5+(-8)
$$
and since addition is also commutative you get
$$
(-8)+5
$$
Hope the long text will not scare you away and I could help a bit
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add a comment |
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With multiplication and division alone (no addition or subtraction) they are associative with respect to one another, BUT division itself is NOT commutative.
So basically you can do the multiplication or division in either order, BUT you must respect which way you interpret the inputs to the left or right of the operator's sign. If you flip this it has the effect of flipping the inputs which is equal precisely when the operator is commutative. As you noted, with division, it is not commutative.
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No, as Mark Bennet explained.
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– Henning Makholm
17 hours ago
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Okay if you vote so.
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– marshal craft
17 hours ago
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Mark Bennets answer is incorrect. You CAN read from either right or left, provided you interpret the same operator symbol the same. So input on left of operator goes to numerator, etc. If you don't change convention you can read the combined expression from either left or right. It's indisputable that there are two levels of right and left. This is not addressed. Yet fundamental to the combined notion of associative and commutative.
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– marshal craft
16 hours ago
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You clearly lack the prerequisites to answer this question. That is all you're displaying for all to see?
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– marshal craft
16 hours ago
2
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Mark Bennet's answer shows that $adiv (btimes c)$ is not generally equal to $(adiv b)times c$. His arithmetic is completely correct. You seem to be taking a view in which the operators $times$ and $div$ are not binary operators but are merely flags of some kind within an $n$-tuple of factors. I don't think that view is widely shared.
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– David K
14 hours ago
|
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9 Answers
9
active
oldest
votes
9 Answers
9
active
oldest
votes
active
oldest
votes
active
oldest
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$begingroup$
Notice that you have always $div 8$, no matter the order of the other terms. You don't divide by a different number. It might help to think $div c$ as a multiplication with $d=1/c$. Then everything would look easier:
$$atimes bdiv c=atimes btimes d=atimes dtimes b=adiv ctimes b$$
$endgroup$
add a comment |
$begingroup$
Notice that you have always $div 8$, no matter the order of the other terms. You don't divide by a different number. It might help to think $div c$ as a multiplication with $d=1/c$. Then everything would look easier:
$$atimes bdiv c=atimes btimes d=atimes dtimes b=adiv ctimes b$$
$endgroup$
add a comment |
$begingroup$
Notice that you have always $div 8$, no matter the order of the other terms. You don't divide by a different number. It might help to think $div c$ as a multiplication with $d=1/c$. Then everything would look easier:
$$atimes bdiv c=atimes btimes d=atimes dtimes b=adiv ctimes b$$
$endgroup$
Notice that you have always $div 8$, no matter the order of the other terms. You don't divide by a different number. It might help to think $div c$ as a multiplication with $d=1/c$. Then everything would look easier:
$$atimes bdiv c=atimes btimes d=atimes dtimes b=adiv ctimes b$$
answered 21 hours ago
AndreiAndrei
12.3k21128
12.3k21128
add a comment |
add a comment |
$begingroup$
Hope this makes sense.
$$ atimes b ÷ c $$
$$=atimesdfrac{b}{c}$$
$$=atimes btimesdfrac{1}{c}$$
$$=(atimesdfrac{1}{c})times b$$
$$=dfrac{a}{c}times b$$
$$=a÷ctimes b$$
New contributor
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$begingroup$
The first step is not elementary. The literal interpretation of the order of operations in $atimes b div c$ is $(atimes b) div c$. what gives us the ability to take the $b$ out of the first operation and put it into $frac bc$? I think this answer would be better if it went straight from $atimes b div c$ to $atimes b times frac1c$.
$endgroup$
– David K
14 hours ago
add a comment |
$begingroup$
Hope this makes sense.
$$ atimes b ÷ c $$
$$=atimesdfrac{b}{c}$$
$$=atimes btimesdfrac{1}{c}$$
$$=(atimesdfrac{1}{c})times b$$
$$=dfrac{a}{c}times b$$
$$=a÷ctimes b$$
New contributor
$endgroup$
$begingroup$
The first step is not elementary. The literal interpretation of the order of operations in $atimes b div c$ is $(atimes b) div c$. what gives us the ability to take the $b$ out of the first operation and put it into $frac bc$? I think this answer would be better if it went straight from $atimes b div c$ to $atimes b times frac1c$.
$endgroup$
– David K
14 hours ago
add a comment |
$begingroup$
Hope this makes sense.
$$ atimes b ÷ c $$
$$=atimesdfrac{b}{c}$$
$$=atimes btimesdfrac{1}{c}$$
$$=(atimesdfrac{1}{c})times b$$
$$=dfrac{a}{c}times b$$
$$=a÷ctimes b$$
New contributor
$endgroup$
Hope this makes sense.
$$ atimes b ÷ c $$
$$=atimesdfrac{b}{c}$$
$$=atimes btimesdfrac{1}{c}$$
$$=(atimesdfrac{1}{c})times b$$
$$=dfrac{a}{c}times b$$
$$=a÷ctimes b$$
New contributor
New contributor
answered 21 hours ago
AdityaAditya
1594
1594
New contributor
New contributor
$begingroup$
The first step is not elementary. The literal interpretation of the order of operations in $atimes b div c$ is $(atimes b) div c$. what gives us the ability to take the $b$ out of the first operation and put it into $frac bc$? I think this answer would be better if it went straight from $atimes b div c$ to $atimes b times frac1c$.
$endgroup$
– David K
14 hours ago
add a comment |
$begingroup$
The first step is not elementary. The literal interpretation of the order of operations in $atimes b div c$ is $(atimes b) div c$. what gives us the ability to take the $b$ out of the first operation and put it into $frac bc$? I think this answer would be better if it went straight from $atimes b div c$ to $atimes b times frac1c$.
$endgroup$
– David K
14 hours ago
$begingroup$
The first step is not elementary. The literal interpretation of the order of operations in $atimes b div c$ is $(atimes b) div c$. what gives us the ability to take the $b$ out of the first operation and put it into $frac bc$? I think this answer would be better if it went straight from $atimes b div c$ to $atimes b times frac1c$.
$endgroup$
– David K
14 hours ago
$begingroup$
The first step is not elementary. The literal interpretation of the order of operations in $atimes b div c$ is $(atimes b) div c$. what gives us the ability to take the $b$ out of the first operation and put it into $frac bc$? I think this answer would be better if it went straight from $atimes b div c$ to $atimes b times frac1c$.
$endgroup$
– David K
14 hours ago
add a comment |
$begingroup$
Because division is the inverse of multiplication, that is: $$X div Y =Xcdot frac 1Y$$
So you have: $$56cdot 100 cdot frac18 =56cdotfrac18cdot100$$
Which is obvious.
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$begingroup$
While it is true that "division is the inverse of multiplication" the emphasis should be on what follows from it: That division is equivalent to multiplication with the inverse and can be replaced by it.
$endgroup$
– Peter A. Schneider
20 hours ago
add a comment |
$begingroup$
Because division is the inverse of multiplication, that is: $$X div Y =Xcdot frac 1Y$$
So you have: $$56cdot 100 cdot frac18 =56cdotfrac18cdot100$$
Which is obvious.
$endgroup$
$begingroup$
While it is true that "division is the inverse of multiplication" the emphasis should be on what follows from it: That division is equivalent to multiplication with the inverse and can be replaced by it.
$endgroup$
– Peter A. Schneider
20 hours ago
add a comment |
$begingroup$
Because division is the inverse of multiplication, that is: $$X div Y =Xcdot frac 1Y$$
So you have: $$56cdot 100 cdot frac18 =56cdotfrac18cdot100$$
Which is obvious.
$endgroup$
Because division is the inverse of multiplication, that is: $$X div Y =Xcdot frac 1Y$$
So you have: $$56cdot 100 cdot frac18 =56cdotfrac18cdot100$$
Which is obvious.
answered 21 hours ago
Rhys HughesRhys Hughes
6,7551530
6,7551530
$begingroup$
While it is true that "division is the inverse of multiplication" the emphasis should be on what follows from it: That division is equivalent to multiplication with the inverse and can be replaced by it.
$endgroup$
– Peter A. Schneider
20 hours ago
add a comment |
$begingroup$
While it is true that "division is the inverse of multiplication" the emphasis should be on what follows from it: That division is equivalent to multiplication with the inverse and can be replaced by it.
$endgroup$
– Peter A. Schneider
20 hours ago
$begingroup$
While it is true that "division is the inverse of multiplication" the emphasis should be on what follows from it: That division is equivalent to multiplication with the inverse and can be replaced by it.
$endgroup$
– Peter A. Schneider
20 hours ago
$begingroup$
While it is true that "division is the inverse of multiplication" the emphasis should be on what follows from it: That division is equivalent to multiplication with the inverse and can be replaced by it.
$endgroup$
– Peter A. Schneider
20 hours ago
add a comment |
$begingroup$
Others have answered the direct question, in that multiplication is commutative and that applies also to multiplication by a reciprocal (the equivalent of division). However there is an issue here with associativity and division which I think is worth mentioning. This has to do with the order in which operations are carried out.
So $adiv b times c$ is being interpreted from left to right as $(adiv b)times c=cfrac {ac}b$, but done from right to left $adiv (btimes c)=cfrac a{bc}$ and the two results are not the same.
Likewise with $adiv b div c$ we have $(adiv b)div c=cfrac a{bc} neq adiv (bdiv c)=cfrac {ac}b$.
So the conventional assumption that multiplication and division are operations of equal status and are carried out from left to right does make a difference in these cases and changes the result. This may be what is feeding your intuition that there is a potential problem with order.
$endgroup$
add a comment |
$begingroup$
Others have answered the direct question, in that multiplication is commutative and that applies also to multiplication by a reciprocal (the equivalent of division). However there is an issue here with associativity and division which I think is worth mentioning. This has to do with the order in which operations are carried out.
So $adiv b times c$ is being interpreted from left to right as $(adiv b)times c=cfrac {ac}b$, but done from right to left $adiv (btimes c)=cfrac a{bc}$ and the two results are not the same.
Likewise with $adiv b div c$ we have $(adiv b)div c=cfrac a{bc} neq adiv (bdiv c)=cfrac {ac}b$.
So the conventional assumption that multiplication and division are operations of equal status and are carried out from left to right does make a difference in these cases and changes the result. This may be what is feeding your intuition that there is a potential problem with order.
$endgroup$
add a comment |
$begingroup$
Others have answered the direct question, in that multiplication is commutative and that applies also to multiplication by a reciprocal (the equivalent of division). However there is an issue here with associativity and division which I think is worth mentioning. This has to do with the order in which operations are carried out.
So $adiv b times c$ is being interpreted from left to right as $(adiv b)times c=cfrac {ac}b$, but done from right to left $adiv (btimes c)=cfrac a{bc}$ and the two results are not the same.
Likewise with $adiv b div c$ we have $(adiv b)div c=cfrac a{bc} neq adiv (bdiv c)=cfrac {ac}b$.
So the conventional assumption that multiplication and division are operations of equal status and are carried out from left to right does make a difference in these cases and changes the result. This may be what is feeding your intuition that there is a potential problem with order.
$endgroup$
Others have answered the direct question, in that multiplication is commutative and that applies also to multiplication by a reciprocal (the equivalent of division). However there is an issue here with associativity and division which I think is worth mentioning. This has to do with the order in which operations are carried out.
So $adiv b times c$ is being interpreted from left to right as $(adiv b)times c=cfrac {ac}b$, but done from right to left $adiv (btimes c)=cfrac a{bc}$ and the two results are not the same.
Likewise with $adiv b div c$ we have $(adiv b)div c=cfrac a{bc} neq adiv (bdiv c)=cfrac {ac}b$.
So the conventional assumption that multiplication and division are operations of equal status and are carried out from left to right does make a difference in these cases and changes the result. This may be what is feeding your intuition that there is a potential problem with order.
answered 21 hours ago
Mark BennetMark Bennet
81.3k983180
81.3k983180
add a comment |
add a comment |
$begingroup$
The non-zero real numbers form an abelian (commutative) group under multiplication. The notation $adiv b$ is shorthand for $ab^{-1}$.
So $ab^{-1}ne ba^{-1}$ but $56times100 times8^{-1}=56times 8^{-1}times 100$
$endgroup$
15
$begingroup$
(-1) this answer is almost certainly nearly useless; the set of people that would ask this question will be nearly disjoint from the set of people that understand this answer
$endgroup$
– DreamConspiracy
19 hours ago
1
$begingroup$
Additionally, it's not even clear he asking over reals, could be over natural numbers in which case there is no group with multiplication as group operation.
$endgroup$
– marshal craft
17 hours ago
add a comment |
$begingroup$
The non-zero real numbers form an abelian (commutative) group under multiplication. The notation $adiv b$ is shorthand for $ab^{-1}$.
So $ab^{-1}ne ba^{-1}$ but $56times100 times8^{-1}=56times 8^{-1}times 100$
$endgroup$
15
$begingroup$
(-1) this answer is almost certainly nearly useless; the set of people that would ask this question will be nearly disjoint from the set of people that understand this answer
$endgroup$
– DreamConspiracy
19 hours ago
1
$begingroup$
Additionally, it's not even clear he asking over reals, could be over natural numbers in which case there is no group with multiplication as group operation.
$endgroup$
– marshal craft
17 hours ago
add a comment |
$begingroup$
The non-zero real numbers form an abelian (commutative) group under multiplication. The notation $adiv b$ is shorthand for $ab^{-1}$.
So $ab^{-1}ne ba^{-1}$ but $56times100 times8^{-1}=56times 8^{-1}times 100$
$endgroup$
The non-zero real numbers form an abelian (commutative) group under multiplication. The notation $adiv b$ is shorthand for $ab^{-1}$.
So $ab^{-1}ne ba^{-1}$ but $56times100 times8^{-1}=56times 8^{-1}times 100$
edited 14 hours ago
Solomon Ucko
1136
1136
answered 21 hours ago
Angela RichardsonAngela Richardson
5,33111733
5,33111733
15
$begingroup$
(-1) this answer is almost certainly nearly useless; the set of people that would ask this question will be nearly disjoint from the set of people that understand this answer
$endgroup$
– DreamConspiracy
19 hours ago
1
$begingroup$
Additionally, it's not even clear he asking over reals, could be over natural numbers in which case there is no group with multiplication as group operation.
$endgroup$
– marshal craft
17 hours ago
add a comment |
15
$begingroup$
(-1) this answer is almost certainly nearly useless; the set of people that would ask this question will be nearly disjoint from the set of people that understand this answer
$endgroup$
– DreamConspiracy
19 hours ago
1
$begingroup$
Additionally, it's not even clear he asking over reals, could be over natural numbers in which case there is no group with multiplication as group operation.
$endgroup$
– marshal craft
17 hours ago
15
15
$begingroup$
(-1) this answer is almost certainly nearly useless; the set of people that would ask this question will be nearly disjoint from the set of people that understand this answer
$endgroup$
– DreamConspiracy
19 hours ago
$begingroup$
(-1) this answer is almost certainly nearly useless; the set of people that would ask this question will be nearly disjoint from the set of people that understand this answer
$endgroup$
– DreamConspiracy
19 hours ago
1
1
$begingroup$
Additionally, it's not even clear he asking over reals, could be over natural numbers in which case there is no group with multiplication as group operation.
$endgroup$
– marshal craft
17 hours ago
$begingroup$
Additionally, it's not even clear he asking over reals, could be over natural numbers in which case there is no group with multiplication as group operation.
$endgroup$
– marshal craft
17 hours ago
add a comment |
$begingroup$
The exact same situation occurs with addition and subtraction instead of multiplication and division. Do you find it hard to justify rewriting say $a+b-c$ to $a-c+b$? And if not, which laws are you using to justify this? I would say associativity, commutativity, and the special equivalence $x-y=x+(-y)$ (which can be construed to be the definition of subtraction, although historically and in teaching subtraction is introduced before introducing negative numbers). In detail one has inserting parentheses to make explicit the one implied by convention)
$$eqalign{
a+b-c &= (a+b)-c \&= (a+b)+(-c) \&= a+(b+(-c)) \&
= a+((-c)+b) \&= (a+(-c))+b \&= (a-c)+b &= a-c+b}
$$
By perfect analogy, let us do that with '$times$' replacing '$+$', and '$div$' replacing '$-$':
$$eqalign{
atimes bdiv c &= (atimes b)div c \&= (atimes b)times (div c) \&= atimes (btimes (div c)) \&
= atimes ((div c)times b) \&= (atimes (div c))times b \&= (adiv c)times b &= adiv ctimes b}
$$
Note how I just invented the unary use of '$div$', where $div x$ of course means the multiplicative inverse of$~x$, just as $-x$ means its additive inverse. Now why did nobody think of that before? It could have avoided the ugliness of$~x^{-1}$, which has to borrow exponentiation and additive inverse to get multiplicative inverse.
$endgroup$
add a comment |
$begingroup$
The exact same situation occurs with addition and subtraction instead of multiplication and division. Do you find it hard to justify rewriting say $a+b-c$ to $a-c+b$? And if not, which laws are you using to justify this? I would say associativity, commutativity, and the special equivalence $x-y=x+(-y)$ (which can be construed to be the definition of subtraction, although historically and in teaching subtraction is introduced before introducing negative numbers). In detail one has inserting parentheses to make explicit the one implied by convention)
$$eqalign{
a+b-c &= (a+b)-c \&= (a+b)+(-c) \&= a+(b+(-c)) \&
= a+((-c)+b) \&= (a+(-c))+b \&= (a-c)+b &= a-c+b}
$$
By perfect analogy, let us do that with '$times$' replacing '$+$', and '$div$' replacing '$-$':
$$eqalign{
atimes bdiv c &= (atimes b)div c \&= (atimes b)times (div c) \&= atimes (btimes (div c)) \&
= atimes ((div c)times b) \&= (atimes (div c))times b \&= (adiv c)times b &= adiv ctimes b}
$$
Note how I just invented the unary use of '$div$', where $div x$ of course means the multiplicative inverse of$~x$, just as $-x$ means its additive inverse. Now why did nobody think of that before? It could have avoided the ugliness of$~x^{-1}$, which has to borrow exponentiation and additive inverse to get multiplicative inverse.
$endgroup$
add a comment |
$begingroup$
The exact same situation occurs with addition and subtraction instead of multiplication and division. Do you find it hard to justify rewriting say $a+b-c$ to $a-c+b$? And if not, which laws are you using to justify this? I would say associativity, commutativity, and the special equivalence $x-y=x+(-y)$ (which can be construed to be the definition of subtraction, although historically and in teaching subtraction is introduced before introducing negative numbers). In detail one has inserting parentheses to make explicit the one implied by convention)
$$eqalign{
a+b-c &= (a+b)-c \&= (a+b)+(-c) \&= a+(b+(-c)) \&
= a+((-c)+b) \&= (a+(-c))+b \&= (a-c)+b &= a-c+b}
$$
By perfect analogy, let us do that with '$times$' replacing '$+$', and '$div$' replacing '$-$':
$$eqalign{
atimes bdiv c &= (atimes b)div c \&= (atimes b)times (div c) \&= atimes (btimes (div c)) \&
= atimes ((div c)times b) \&= (atimes (div c))times b \&= (adiv c)times b &= adiv ctimes b}
$$
Note how I just invented the unary use of '$div$', where $div x$ of course means the multiplicative inverse of$~x$, just as $-x$ means its additive inverse. Now why did nobody think of that before? It could have avoided the ugliness of$~x^{-1}$, which has to borrow exponentiation and additive inverse to get multiplicative inverse.
$endgroup$
The exact same situation occurs with addition and subtraction instead of multiplication and division. Do you find it hard to justify rewriting say $a+b-c$ to $a-c+b$? And if not, which laws are you using to justify this? I would say associativity, commutativity, and the special equivalence $x-y=x+(-y)$ (which can be construed to be the definition of subtraction, although historically and in teaching subtraction is introduced before introducing negative numbers). In detail one has inserting parentheses to make explicit the one implied by convention)
$$eqalign{
a+b-c &= (a+b)-c \&= (a+b)+(-c) \&= a+(b+(-c)) \&
= a+((-c)+b) \&= (a+(-c))+b \&= (a-c)+b &= a-c+b}
$$
By perfect analogy, let us do that with '$times$' replacing '$+$', and '$div$' replacing '$-$':
$$eqalign{
atimes bdiv c &= (atimes b)div c \&= (atimes b)times (div c) \&= atimes (btimes (div c)) \&
= atimes ((div c)times b) \&= (atimes (div c))times b \&= (adiv c)times b &= adiv ctimes b}
$$
Note how I just invented the unary use of '$div$', where $div x$ of course means the multiplicative inverse of$~x$, just as $-x$ means its additive inverse. Now why did nobody think of that before? It could have avoided the ugliness of$~x^{-1}$, which has to borrow exponentiation and additive inverse to get multiplicative inverse.
answered 15 hours ago
Marc van LeeuwenMarc van Leeuwen
87.4k5109224
87.4k5109224
add a comment |
add a comment |
$begingroup$
Start with 56 × 100 ÷ 8.
Swap the first two factors which is definitely allowed: 100 × 56 ÷ 8.
Now put parentheses around part of it: 100 × (56 ÷ 8).
Again, swap: (56 ÷ 8) × 100.
Take off the parentheses: 56 ÷ 8 × 100.
And there you go! transforming one into the other using only multiplicative commutation (plus the fact that one of the factors commutated can be a product, not just a simple value).
New contributor
$endgroup$
2
$begingroup$
Now explain why putting parentheses around the last two terms in $100times56div8$ is OK but putting parentheses around the last two terms in $56div8times100$ is not OK. For that matter, why is it OK to swap the first two terms in $100times56div8$ but not the last two terms of $56div8times100$?
$endgroup$
– David K
14 hours ago
$begingroup$
The short answer to adding the parentheses is, "because it doesn't change the outcome". But you're right, a more rigorous explanation is needed, and I don't have one. To swap the last two terms of 56 ÷ 8 × 100 does require parentheses around them, and if they aren't already there, they can't be arbitrarily added without changing the whole thing.
$endgroup$
– Jennifer
14 hours ago
$begingroup$
A formal reason is that $56times100div8$ means $(56times100)div8$ by convention, and $(56times100)div8=(100times56)div8=(100times56)timesfrac18=100times(56timesfrac18)=100times(56div8).$ But that's making it more complicated than necessary. I think it's simpler if we start with $(56times100)div8=(56times100)timesfrac18$.
$endgroup$
– David K
13 hours ago
add a comment |
$begingroup$
Start with 56 × 100 ÷ 8.
Swap the first two factors which is definitely allowed: 100 × 56 ÷ 8.
Now put parentheses around part of it: 100 × (56 ÷ 8).
Again, swap: (56 ÷ 8) × 100.
Take off the parentheses: 56 ÷ 8 × 100.
And there you go! transforming one into the other using only multiplicative commutation (plus the fact that one of the factors commutated can be a product, not just a simple value).
New contributor
$endgroup$
2
$begingroup$
Now explain why putting parentheses around the last two terms in $100times56div8$ is OK but putting parentheses around the last two terms in $56div8times100$ is not OK. For that matter, why is it OK to swap the first two terms in $100times56div8$ but not the last two terms of $56div8times100$?
$endgroup$
– David K
14 hours ago
$begingroup$
The short answer to adding the parentheses is, "because it doesn't change the outcome". But you're right, a more rigorous explanation is needed, and I don't have one. To swap the last two terms of 56 ÷ 8 × 100 does require parentheses around them, and if they aren't already there, they can't be arbitrarily added without changing the whole thing.
$endgroup$
– Jennifer
14 hours ago
$begingroup$
A formal reason is that $56times100div8$ means $(56times100)div8$ by convention, and $(56times100)div8=(100times56)div8=(100times56)timesfrac18=100times(56timesfrac18)=100times(56div8).$ But that's making it more complicated than necessary. I think it's simpler if we start with $(56times100)div8=(56times100)timesfrac18$.
$endgroup$
– David K
13 hours ago
add a comment |
$begingroup$
Start with 56 × 100 ÷ 8.
Swap the first two factors which is definitely allowed: 100 × 56 ÷ 8.
Now put parentheses around part of it: 100 × (56 ÷ 8).
Again, swap: (56 ÷ 8) × 100.
Take off the parentheses: 56 ÷ 8 × 100.
And there you go! transforming one into the other using only multiplicative commutation (plus the fact that one of the factors commutated can be a product, not just a simple value).
New contributor
$endgroup$
Start with 56 × 100 ÷ 8.
Swap the first two factors which is definitely allowed: 100 × 56 ÷ 8.
Now put parentheses around part of it: 100 × (56 ÷ 8).
Again, swap: (56 ÷ 8) × 100.
Take off the parentheses: 56 ÷ 8 × 100.
And there you go! transforming one into the other using only multiplicative commutation (plus the fact that one of the factors commutated can be a product, not just a simple value).
New contributor
New contributor
answered 14 hours ago
JenniferJennifer
111
111
New contributor
New contributor
2
$begingroup$
Now explain why putting parentheses around the last two terms in $100times56div8$ is OK but putting parentheses around the last two terms in $56div8times100$ is not OK. For that matter, why is it OK to swap the first two terms in $100times56div8$ but not the last two terms of $56div8times100$?
$endgroup$
– David K
14 hours ago
$begingroup$
The short answer to adding the parentheses is, "because it doesn't change the outcome". But you're right, a more rigorous explanation is needed, and I don't have one. To swap the last two terms of 56 ÷ 8 × 100 does require parentheses around them, and if they aren't already there, they can't be arbitrarily added without changing the whole thing.
$endgroup$
– Jennifer
14 hours ago
$begingroup$
A formal reason is that $56times100div8$ means $(56times100)div8$ by convention, and $(56times100)div8=(100times56)div8=(100times56)timesfrac18=100times(56timesfrac18)=100times(56div8).$ But that's making it more complicated than necessary. I think it's simpler if we start with $(56times100)div8=(56times100)timesfrac18$.
$endgroup$
– David K
13 hours ago
add a comment |
2
$begingroup$
Now explain why putting parentheses around the last two terms in $100times56div8$ is OK but putting parentheses around the last two terms in $56div8times100$ is not OK. For that matter, why is it OK to swap the first two terms in $100times56div8$ but not the last two terms of $56div8times100$?
$endgroup$
– David K
14 hours ago
$begingroup$
The short answer to adding the parentheses is, "because it doesn't change the outcome". But you're right, a more rigorous explanation is needed, and I don't have one. To swap the last two terms of 56 ÷ 8 × 100 does require parentheses around them, and if they aren't already there, they can't be arbitrarily added without changing the whole thing.
$endgroup$
– Jennifer
14 hours ago
$begingroup$
A formal reason is that $56times100div8$ means $(56times100)div8$ by convention, and $(56times100)div8=(100times56)div8=(100times56)timesfrac18=100times(56timesfrac18)=100times(56div8).$ But that's making it more complicated than necessary. I think it's simpler if we start with $(56times100)div8=(56times100)timesfrac18$.
$endgroup$
– David K
13 hours ago
2
2
$begingroup$
Now explain why putting parentheses around the last two terms in $100times56div8$ is OK but putting parentheses around the last two terms in $56div8times100$ is not OK. For that matter, why is it OK to swap the first two terms in $100times56div8$ but not the last two terms of $56div8times100$?
$endgroup$
– David K
14 hours ago
$begingroup$
Now explain why putting parentheses around the last two terms in $100times56div8$ is OK but putting parentheses around the last two terms in $56div8times100$ is not OK. For that matter, why is it OK to swap the first two terms in $100times56div8$ but not the last two terms of $56div8times100$?
$endgroup$
– David K
14 hours ago
$begingroup$
The short answer to adding the parentheses is, "because it doesn't change the outcome". But you're right, a more rigorous explanation is needed, and I don't have one. To swap the last two terms of 56 ÷ 8 × 100 does require parentheses around them, and if they aren't already there, they can't be arbitrarily added without changing the whole thing.
$endgroup$
– Jennifer
14 hours ago
$begingroup$
The short answer to adding the parentheses is, "because it doesn't change the outcome". But you're right, a more rigorous explanation is needed, and I don't have one. To swap the last two terms of 56 ÷ 8 × 100 does require parentheses around them, and if they aren't already there, they can't be arbitrarily added without changing the whole thing.
$endgroup$
– Jennifer
14 hours ago
$begingroup$
A formal reason is that $56times100div8$ means $(56times100)div8$ by convention, and $(56times100)div8=(100times56)div8=(100times56)timesfrac18=100times(56timesfrac18)=100times(56div8).$ But that's making it more complicated than necessary. I think it's simpler if we start with $(56times100)div8=(56times100)timesfrac18$.
$endgroup$
– David K
13 hours ago
$begingroup$
A formal reason is that $56times100div8$ means $(56times100)div8$ by convention, and $(56times100)div8=(100times56)div8=(100times56)timesfrac18=100times(56timesfrac18)=100times(56div8).$ But that's making it more complicated than necessary. I think it's simpler if we start with $(56times100)div8=(56times100)timesfrac18$.
$endgroup$
– David K
13 hours ago
add a comment |
$begingroup$
Your confusion comes from a fact that is rarely spoken of in school.
The real numbers constitute a so called field where we have two operations, multiplication and addition. These two in their own create a so called group with the given set, that is the real numbers $mathbb{R}$.
This may sound really cryptic but let me clear it up. In a group you need a set $X$ and have one and only one operation $*$, you have a neutral element $e$ fullfilling that it doesnt change the element which is combined with it, all the members of the set can be combined using $*$ which will result in another element of the set $X$ and all the elements have so called inverses that is for each element $a$ in $X$ you can find another element $b$ such that $a*b=e$.
Now this is still a bit formal and cryptic so let us find an example. The whole numbers, also called as the integers $mathbb{Z}$ with usual addition forms a group.
You can add integers and you end up with integers, $0$ serves as the neutral element since any integer plus $0$ is the same integer, unchanged and you also have inverses the so called negative numbers $5 + (-5)=0$. Because of lazyness we usually write this as $5-5=0$ giving the false idea that we also have minus $"-"$ as an operation but that is WRONG.
Another exapmle could be the integers with the operation multiplication. We can multiply any integers together resulting another integer so it looks okay so far, we can find a neutral element, namely $1$ which fullfills the desired property, namely any integer times one is the same integer but we fail to find inverses. You cannot find an integer to multiply, say $5$ with to get the neutral element $1$. So this is NOT a group.
All possible fractions also callet as quotients $mathbb{Q}$ is a group with respect to multiplication (everything looks similar to what stands directly above here) but here you will find inverses. $5$ can be multiplied by $frac{1}{5}$ to get one. But once again this DOES NOT make division an operation. It is tempting to think of $frac{1}{5}$ as one divided by five which may suggest that division is an actual operation in this group but $frac{1}{5}$ is an element.
So after this short tour into abstract algebra let us consider your question again
Remember we are working in $mathbb{R}$, the field of real numbers which means that $mathbb{R}$ is a group with respect to both addition and multiplication. Or even better you could translate $frac{1}{8}$ as the inverse of $8$ with respect to multiplication
$$
frac{56}{8}cdot 100
$$
actually means that take three elements of $mathbb{R}$, namely $frac{1}{8}, 100$ and $56$ and multiply them together and since multiplication is commutative you may do this in any order. Try not to think of division as an operation rather than a way to express elements of this group.
BONUS FACT:
thinking this way also helps debunking mysteries about subtraction
$$
5-8neq 8-5
$$
which is inherently wrong since once again it suggests that minus is an operation. But using the correct way of thinking, that is the minus sign is "glued" to a number as an attribute so to say you get
$$
5+(-8)
$$
and since addition is also commutative you get
$$
(-8)+5
$$
Hope the long text will not scare you away and I could help a bit
$endgroup$
add a comment |
$begingroup$
Your confusion comes from a fact that is rarely spoken of in school.
The real numbers constitute a so called field where we have two operations, multiplication and addition. These two in their own create a so called group with the given set, that is the real numbers $mathbb{R}$.
This may sound really cryptic but let me clear it up. In a group you need a set $X$ and have one and only one operation $*$, you have a neutral element $e$ fullfilling that it doesnt change the element which is combined with it, all the members of the set can be combined using $*$ which will result in another element of the set $X$ and all the elements have so called inverses that is for each element $a$ in $X$ you can find another element $b$ such that $a*b=e$.
Now this is still a bit formal and cryptic so let us find an example. The whole numbers, also called as the integers $mathbb{Z}$ with usual addition forms a group.
You can add integers and you end up with integers, $0$ serves as the neutral element since any integer plus $0$ is the same integer, unchanged and you also have inverses the so called negative numbers $5 + (-5)=0$. Because of lazyness we usually write this as $5-5=0$ giving the false idea that we also have minus $"-"$ as an operation but that is WRONG.
Another exapmle could be the integers with the operation multiplication. We can multiply any integers together resulting another integer so it looks okay so far, we can find a neutral element, namely $1$ which fullfills the desired property, namely any integer times one is the same integer but we fail to find inverses. You cannot find an integer to multiply, say $5$ with to get the neutral element $1$. So this is NOT a group.
All possible fractions also callet as quotients $mathbb{Q}$ is a group with respect to multiplication (everything looks similar to what stands directly above here) but here you will find inverses. $5$ can be multiplied by $frac{1}{5}$ to get one. But once again this DOES NOT make division an operation. It is tempting to think of $frac{1}{5}$ as one divided by five which may suggest that division is an actual operation in this group but $frac{1}{5}$ is an element.
So after this short tour into abstract algebra let us consider your question again
Remember we are working in $mathbb{R}$, the field of real numbers which means that $mathbb{R}$ is a group with respect to both addition and multiplication. Or even better you could translate $frac{1}{8}$ as the inverse of $8$ with respect to multiplication
$$
frac{56}{8}cdot 100
$$
actually means that take three elements of $mathbb{R}$, namely $frac{1}{8}, 100$ and $56$ and multiply them together and since multiplication is commutative you may do this in any order. Try not to think of division as an operation rather than a way to express elements of this group.
BONUS FACT:
thinking this way also helps debunking mysteries about subtraction
$$
5-8neq 8-5
$$
which is inherently wrong since once again it suggests that minus is an operation. But using the correct way of thinking, that is the minus sign is "glued" to a number as an attribute so to say you get
$$
5+(-8)
$$
and since addition is also commutative you get
$$
(-8)+5
$$
Hope the long text will not scare you away and I could help a bit
$endgroup$
add a comment |
$begingroup$
Your confusion comes from a fact that is rarely spoken of in school.
The real numbers constitute a so called field where we have two operations, multiplication and addition. These two in their own create a so called group with the given set, that is the real numbers $mathbb{R}$.
This may sound really cryptic but let me clear it up. In a group you need a set $X$ and have one and only one operation $*$, you have a neutral element $e$ fullfilling that it doesnt change the element which is combined with it, all the members of the set can be combined using $*$ which will result in another element of the set $X$ and all the elements have so called inverses that is for each element $a$ in $X$ you can find another element $b$ such that $a*b=e$.
Now this is still a bit formal and cryptic so let us find an example. The whole numbers, also called as the integers $mathbb{Z}$ with usual addition forms a group.
You can add integers and you end up with integers, $0$ serves as the neutral element since any integer plus $0$ is the same integer, unchanged and you also have inverses the so called negative numbers $5 + (-5)=0$. Because of lazyness we usually write this as $5-5=0$ giving the false idea that we also have minus $"-"$ as an operation but that is WRONG.
Another exapmle could be the integers with the operation multiplication. We can multiply any integers together resulting another integer so it looks okay so far, we can find a neutral element, namely $1$ which fullfills the desired property, namely any integer times one is the same integer but we fail to find inverses. You cannot find an integer to multiply, say $5$ with to get the neutral element $1$. So this is NOT a group.
All possible fractions also callet as quotients $mathbb{Q}$ is a group with respect to multiplication (everything looks similar to what stands directly above here) but here you will find inverses. $5$ can be multiplied by $frac{1}{5}$ to get one. But once again this DOES NOT make division an operation. It is tempting to think of $frac{1}{5}$ as one divided by five which may suggest that division is an actual operation in this group but $frac{1}{5}$ is an element.
So after this short tour into abstract algebra let us consider your question again
Remember we are working in $mathbb{R}$, the field of real numbers which means that $mathbb{R}$ is a group with respect to both addition and multiplication. Or even better you could translate $frac{1}{8}$ as the inverse of $8$ with respect to multiplication
$$
frac{56}{8}cdot 100
$$
actually means that take three elements of $mathbb{R}$, namely $frac{1}{8}, 100$ and $56$ and multiply them together and since multiplication is commutative you may do this in any order. Try not to think of division as an operation rather than a way to express elements of this group.
BONUS FACT:
thinking this way also helps debunking mysteries about subtraction
$$
5-8neq 8-5
$$
which is inherently wrong since once again it suggests that minus is an operation. But using the correct way of thinking, that is the minus sign is "glued" to a number as an attribute so to say you get
$$
5+(-8)
$$
and since addition is also commutative you get
$$
(-8)+5
$$
Hope the long text will not scare you away and I could help a bit
$endgroup$
Your confusion comes from a fact that is rarely spoken of in school.
The real numbers constitute a so called field where we have two operations, multiplication and addition. These two in their own create a so called group with the given set, that is the real numbers $mathbb{R}$.
This may sound really cryptic but let me clear it up. In a group you need a set $X$ and have one and only one operation $*$, you have a neutral element $e$ fullfilling that it doesnt change the element which is combined with it, all the members of the set can be combined using $*$ which will result in another element of the set $X$ and all the elements have so called inverses that is for each element $a$ in $X$ you can find another element $b$ such that $a*b=e$.
Now this is still a bit formal and cryptic so let us find an example. The whole numbers, also called as the integers $mathbb{Z}$ with usual addition forms a group.
You can add integers and you end up with integers, $0$ serves as the neutral element since any integer plus $0$ is the same integer, unchanged and you also have inverses the so called negative numbers $5 + (-5)=0$. Because of lazyness we usually write this as $5-5=0$ giving the false idea that we also have minus $"-"$ as an operation but that is WRONG.
Another exapmle could be the integers with the operation multiplication. We can multiply any integers together resulting another integer so it looks okay so far, we can find a neutral element, namely $1$ which fullfills the desired property, namely any integer times one is the same integer but we fail to find inverses. You cannot find an integer to multiply, say $5$ with to get the neutral element $1$. So this is NOT a group.
All possible fractions also callet as quotients $mathbb{Q}$ is a group with respect to multiplication (everything looks similar to what stands directly above here) but here you will find inverses. $5$ can be multiplied by $frac{1}{5}$ to get one. But once again this DOES NOT make division an operation. It is tempting to think of $frac{1}{5}$ as one divided by five which may suggest that division is an actual operation in this group but $frac{1}{5}$ is an element.
So after this short tour into abstract algebra let us consider your question again
Remember we are working in $mathbb{R}$, the field of real numbers which means that $mathbb{R}$ is a group with respect to both addition and multiplication. Or even better you could translate $frac{1}{8}$ as the inverse of $8$ with respect to multiplication
$$
frac{56}{8}cdot 100
$$
actually means that take three elements of $mathbb{R}$, namely $frac{1}{8}, 100$ and $56$ and multiply them together and since multiplication is commutative you may do this in any order. Try not to think of division as an operation rather than a way to express elements of this group.
BONUS FACT:
thinking this way also helps debunking mysteries about subtraction
$$
5-8neq 8-5
$$
which is inherently wrong since once again it suggests that minus is an operation. But using the correct way of thinking, that is the minus sign is "glued" to a number as an attribute so to say you get
$$
5+(-8)
$$
and since addition is also commutative you get
$$
(-8)+5
$$
Hope the long text will not scare you away and I could help a bit
answered 13 hours ago
Vinyl_coat_jawaVinyl_coat_jawa
2,3011028
2,3011028
add a comment |
add a comment |
$begingroup$
With multiplication and division alone (no addition or subtraction) they are associative with respect to one another, BUT division itself is NOT commutative.
So basically you can do the multiplication or division in either order, BUT you must respect which way you interpret the inputs to the left or right of the operator's sign. If you flip this it has the effect of flipping the inputs which is equal precisely when the operator is commutative. As you noted, with division, it is not commutative.
$endgroup$
$begingroup$
No, as Mark Bennet explained.
$endgroup$
– Henning Makholm
17 hours ago
$begingroup$
Okay if you vote so.
$endgroup$
– marshal craft
17 hours ago
$begingroup$
Mark Bennets answer is incorrect. You CAN read from either right or left, provided you interpret the same operator symbol the same. So input on left of operator goes to numerator, etc. If you don't change convention you can read the combined expression from either left or right. It's indisputable that there are two levels of right and left. This is not addressed. Yet fundamental to the combined notion of associative and commutative.
$endgroup$
– marshal craft
16 hours ago
$begingroup$
You clearly lack the prerequisites to answer this question. That is all you're displaying for all to see?
$endgroup$
– marshal craft
16 hours ago
2
$begingroup$
Mark Bennet's answer shows that $adiv (btimes c)$ is not generally equal to $(adiv b)times c$. His arithmetic is completely correct. You seem to be taking a view in which the operators $times$ and $div$ are not binary operators but are merely flags of some kind within an $n$-tuple of factors. I don't think that view is widely shared.
$endgroup$
– David K
14 hours ago
|
show 6 more comments
$begingroup$
With multiplication and division alone (no addition or subtraction) they are associative with respect to one another, BUT division itself is NOT commutative.
So basically you can do the multiplication or division in either order, BUT you must respect which way you interpret the inputs to the left or right of the operator's sign. If you flip this it has the effect of flipping the inputs which is equal precisely when the operator is commutative. As you noted, with division, it is not commutative.
$endgroup$
$begingroup$
No, as Mark Bennet explained.
$endgroup$
– Henning Makholm
17 hours ago
$begingroup$
Okay if you vote so.
$endgroup$
– marshal craft
17 hours ago
$begingroup$
Mark Bennets answer is incorrect. You CAN read from either right or left, provided you interpret the same operator symbol the same. So input on left of operator goes to numerator, etc. If you don't change convention you can read the combined expression from either left or right. It's indisputable that there are two levels of right and left. This is not addressed. Yet fundamental to the combined notion of associative and commutative.
$endgroup$
– marshal craft
16 hours ago
$begingroup$
You clearly lack the prerequisites to answer this question. That is all you're displaying for all to see?
$endgroup$
– marshal craft
16 hours ago
2
$begingroup$
Mark Bennet's answer shows that $adiv (btimes c)$ is not generally equal to $(adiv b)times c$. His arithmetic is completely correct. You seem to be taking a view in which the operators $times$ and $div$ are not binary operators but are merely flags of some kind within an $n$-tuple of factors. I don't think that view is widely shared.
$endgroup$
– David K
14 hours ago
|
show 6 more comments
$begingroup$
With multiplication and division alone (no addition or subtraction) they are associative with respect to one another, BUT division itself is NOT commutative.
So basically you can do the multiplication or division in either order, BUT you must respect which way you interpret the inputs to the left or right of the operator's sign. If you flip this it has the effect of flipping the inputs which is equal precisely when the operator is commutative. As you noted, with division, it is not commutative.
$endgroup$
With multiplication and division alone (no addition or subtraction) they are associative with respect to one another, BUT division itself is NOT commutative.
So basically you can do the multiplication or division in either order, BUT you must respect which way you interpret the inputs to the left or right of the operator's sign. If you flip this it has the effect of flipping the inputs which is equal precisely when the operator is commutative. As you noted, with division, it is not commutative.
edited 17 hours ago
answered 17 hours ago
marshal craftmarshal craft
709414
709414
$begingroup$
No, as Mark Bennet explained.
$endgroup$
– Henning Makholm
17 hours ago
$begingroup$
Okay if you vote so.
$endgroup$
– marshal craft
17 hours ago
$begingroup$
Mark Bennets answer is incorrect. You CAN read from either right or left, provided you interpret the same operator symbol the same. So input on left of operator goes to numerator, etc. If you don't change convention you can read the combined expression from either left or right. It's indisputable that there are two levels of right and left. This is not addressed. Yet fundamental to the combined notion of associative and commutative.
$endgroup$
– marshal craft
16 hours ago
$begingroup$
You clearly lack the prerequisites to answer this question. That is all you're displaying for all to see?
$endgroup$
– marshal craft
16 hours ago
2
$begingroup$
Mark Bennet's answer shows that $adiv (btimes c)$ is not generally equal to $(adiv b)times c$. His arithmetic is completely correct. You seem to be taking a view in which the operators $times$ and $div$ are not binary operators but are merely flags of some kind within an $n$-tuple of factors. I don't think that view is widely shared.
$endgroup$
– David K
14 hours ago
|
show 6 more comments
$begingroup$
No, as Mark Bennet explained.
$endgroup$
– Henning Makholm
17 hours ago
$begingroup$
Okay if you vote so.
$endgroup$
– marshal craft
17 hours ago
$begingroup$
Mark Bennets answer is incorrect. You CAN read from either right or left, provided you interpret the same operator symbol the same. So input on left of operator goes to numerator, etc. If you don't change convention you can read the combined expression from either left or right. It's indisputable that there are two levels of right and left. This is not addressed. Yet fundamental to the combined notion of associative and commutative.
$endgroup$
– marshal craft
16 hours ago
$begingroup$
You clearly lack the prerequisites to answer this question. That is all you're displaying for all to see?
$endgroup$
– marshal craft
16 hours ago
2
$begingroup$
Mark Bennet's answer shows that $adiv (btimes c)$ is not generally equal to $(adiv b)times c$. His arithmetic is completely correct. You seem to be taking a view in which the operators $times$ and $div$ are not binary operators but are merely flags of some kind within an $n$-tuple of factors. I don't think that view is widely shared.
$endgroup$
– David K
14 hours ago
$begingroup$
No, as Mark Bennet explained.
$endgroup$
– Henning Makholm
17 hours ago
$begingroup$
No, as Mark Bennet explained.
$endgroup$
– Henning Makholm
17 hours ago
$begingroup$
Okay if you vote so.
$endgroup$
– marshal craft
17 hours ago
$begingroup$
Okay if you vote so.
$endgroup$
– marshal craft
17 hours ago
$begingroup$
Mark Bennets answer is incorrect. You CAN read from either right or left, provided you interpret the same operator symbol the same. So input on left of operator goes to numerator, etc. If you don't change convention you can read the combined expression from either left or right. It's indisputable that there are two levels of right and left. This is not addressed. Yet fundamental to the combined notion of associative and commutative.
$endgroup$
– marshal craft
16 hours ago
$begingroup$
Mark Bennets answer is incorrect. You CAN read from either right or left, provided you interpret the same operator symbol the same. So input on left of operator goes to numerator, etc. If you don't change convention you can read the combined expression from either left or right. It's indisputable that there are two levels of right and left. This is not addressed. Yet fundamental to the combined notion of associative and commutative.
$endgroup$
– marshal craft
16 hours ago
$begingroup$
You clearly lack the prerequisites to answer this question. That is all you're displaying for all to see?
$endgroup$
– marshal craft
16 hours ago
$begingroup$
You clearly lack the prerequisites to answer this question. That is all you're displaying for all to see?
$endgroup$
– marshal craft
16 hours ago
2
2
$begingroup$
Mark Bennet's answer shows that $adiv (btimes c)$ is not generally equal to $(adiv b)times c$. His arithmetic is completely correct. You seem to be taking a view in which the operators $times$ and $div$ are not binary operators but are merely flags of some kind within an $n$-tuple of factors. I don't think that view is widely shared.
$endgroup$
– David K
14 hours ago
$begingroup$
Mark Bennet's answer shows that $adiv (btimes c)$ is not generally equal to $(adiv b)times c$. His arithmetic is completely correct. You seem to be taking a view in which the operators $times$ and $div$ are not binary operators but are merely flags of some kind within an $n$-tuple of factors. I don't think that view is widely shared.
$endgroup$
– David K
14 hours ago
|
show 6 more comments
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$begingroup$
@marshalcraft: There is no reasonable sense in which multiplication and division are associative "together" -- as Mark Bennet's answer points out, $a/(bcdot c) ne (a/b)cdot c$ in general.
$endgroup$
– Henning Makholm
15 hours ago
$begingroup$
Indeed its an error on my part.
$endgroup$
– marshal craft
15 hours ago
$begingroup$
So I need to say is multiplication is commutative and associative, division is not associative and not commutative. But when you compute the result of division you remove the division and thus can then proceed to use multiplicative property of commutivity and associativity. K.
$endgroup$
– marshal craft
14 hours ago
$begingroup$
To would-be editors: please, please do not convert the $div$ operators in this question into the fractional form $frac ab.$ It changes the question completely.
$endgroup$
– David K
14 hours ago
$begingroup$
Also due to the non commutivity, and non associativity of division, the notation used above is decidably ambiguous. That means there are a number of equally valid ways to interpret expressions ( this is why different calculators can give different results). You can explore the possibilities. In higer level math fractions, parents, etc. are used and there is no ambiguity.
$endgroup$
– marshal craft
13 hours ago