How does the squeeze theorem imply that $lim_{thetato 0}sintheta=0$ and $lim_{thetato 0}costheta=1$?
$begingroup$
I'm trying to understand how the sandwich/ squeeze theorem establishes that $lim_{thetato 0}sintheta = 0$ and $lim_{thetato 0}costheta = 1$.
Mainly, i'm trying to understand this logic, rather than trying to answer an assignment question.
calculus limits
$endgroup$
add a comment |
$begingroup$
I'm trying to understand how the sandwich/ squeeze theorem establishes that $lim_{thetato 0}sintheta = 0$ and $lim_{thetato 0}costheta = 1$.
Mainly, i'm trying to understand this logic, rather than trying to answer an assignment question.
calculus limits
$endgroup$
1
$begingroup$
Are you already equipped with the series representation of $sin theta$ and $cos theta$ and their approximation by Taylor polynomials? Because first you need an appropriate approximation of these functions near $0$ and then we could check how the sandwich trick works.
$endgroup$
– trancelocation
21 hours ago
add a comment |
$begingroup$
I'm trying to understand how the sandwich/ squeeze theorem establishes that $lim_{thetato 0}sintheta = 0$ and $lim_{thetato 0}costheta = 1$.
Mainly, i'm trying to understand this logic, rather than trying to answer an assignment question.
calculus limits
$endgroup$
I'm trying to understand how the sandwich/ squeeze theorem establishes that $lim_{thetato 0}sintheta = 0$ and $lim_{thetato 0}costheta = 1$.
Mainly, i'm trying to understand this logic, rather than trying to answer an assignment question.
calculus limits
calculus limits
edited 18 hours ago
Asaf Karagila♦
305k33435765
305k33435765
asked 21 hours ago
JamesJames
163
163
1
$begingroup$
Are you already equipped with the series representation of $sin theta$ and $cos theta$ and their approximation by Taylor polynomials? Because first you need an appropriate approximation of these functions near $0$ and then we could check how the sandwich trick works.
$endgroup$
– trancelocation
21 hours ago
add a comment |
1
$begingroup$
Are you already equipped with the series representation of $sin theta$ and $cos theta$ and their approximation by Taylor polynomials? Because first you need an appropriate approximation of these functions near $0$ and then we could check how the sandwich trick works.
$endgroup$
– trancelocation
21 hours ago
1
1
$begingroup$
Are you already equipped with the series representation of $sin theta$ and $cos theta$ and their approximation by Taylor polynomials? Because first you need an appropriate approximation of these functions near $0$ and then we could check how the sandwich trick works.
$endgroup$
– trancelocation
21 hours ago
$begingroup$
Are you already equipped with the series representation of $sin theta$ and $cos theta$ and their approximation by Taylor polynomials? Because first you need an appropriate approximation of these functions near $0$ and then we could check how the sandwich trick works.
$endgroup$
– trancelocation
21 hours ago
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
The idea behind sandwich/squeeze theorem is intuitive enough. If values of a function $f$ are lying (sandwiched) between those of $g, h$ and values of $g, h$ tend to a common limit then that of $f$ also tend to the same limit.
We have the fundamental inequality $|sin x|leq |x|$ for $0<|x|<pi/2$ which is the same as $$-|x|leq sin xleq |x|$$ for $0<|x|<pi/2$ and since both $-|x|$ and $|x|$ tend to $0$ as $xto 0$ it follows that $lim_{xto 0}sin x=0$.
For $cos x$ the situation is tricky and we need to use a bit of trigonometry. We have $$cos x=1-2sin^2(x/2)$$ and noting the earlier inequality for $sin $ we get $$1-frac{x^2}{2}leq cos xleq 1$$ And by squeeze we get $lim_{xto 0}cos x=1$.
$endgroup$
add a comment |
$begingroup$
Edit: I corrected some inaccuracies pointed out in the comments.
I try to explain the logic by solving the exercize. First of all, this result is also known as the two policemen theorem because if two policemen are escorting a person between them, and both officers approch arbitrary close to the cell, then the prisoner must also approch arbitrary close to it (notice that the policemen don't need to go to the cell; this means the functions need not to be defined in the limit point). This summarizes the main idea of the theorem.
Now, we know that when $x$ is sufficiently near to $0$ (i.e. the functions must be defined in a neighbourhood of such a point), $-lvert xrvertle sin xle lvert xrvert$ and this holds for any $x$ sufficiently small (actually for any $lvert xrvert $ in $]0,pi/2[$). Since limits preserve the linear order, we have that if limit of $sin x$ exists, then $$lim_{xrightarrow0} -xle lim_{xrightarrow0} sin x le lim_{xrightarrow0} x.$$
But the first and the last limits are $0$, hence limit of $sin x$ exists and is $0$ because this function is eventually bounded both from above and below by functions converging to the same limit, so $sin x$ can't go away.
Generally speaking, you need two functions/policemen who bound the function of which you want to compute the limit, from above and below respectively, in a neighbourhood of the limit point and they must have the same limit at that point.
$endgroup$
3
$begingroup$
Strictly speaking, the officers don't actually have to enter the cell, they just have to approach arbitrarily close to it, and then the prisoner must also approach arbitrarily close to it. :-)
$endgroup$
– Brian Tung
20 hours ago
1
$begingroup$
There are some inaccuracies in this answer. The inequalities $-x leq sin , xleq x$ can hold only for $x geq 0$ so one can only take right hand limits.
$endgroup$
– Kavi Rama Murthy
20 hours ago
$begingroup$
@BrianTung I was so drunk last night that I don't remember exactly what happened with the officers ;-)
$endgroup$
– LBJFS
16 hours ago
$begingroup$
@KaviRamaMurthy I apologize for the errors and thanks for the mistakes you pointed out.
$endgroup$
– LBJFS
16 hours ago
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3119710%2fhow-does-the-squeeze-theorem-imply-that-lim-theta-to-0-sin-theta-0-and-l%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
The idea behind sandwich/squeeze theorem is intuitive enough. If values of a function $f$ are lying (sandwiched) between those of $g, h$ and values of $g, h$ tend to a common limit then that of $f$ also tend to the same limit.
We have the fundamental inequality $|sin x|leq |x|$ for $0<|x|<pi/2$ which is the same as $$-|x|leq sin xleq |x|$$ for $0<|x|<pi/2$ and since both $-|x|$ and $|x|$ tend to $0$ as $xto 0$ it follows that $lim_{xto 0}sin x=0$.
For $cos x$ the situation is tricky and we need to use a bit of trigonometry. We have $$cos x=1-2sin^2(x/2)$$ and noting the earlier inequality for $sin $ we get $$1-frac{x^2}{2}leq cos xleq 1$$ And by squeeze we get $lim_{xto 0}cos x=1$.
$endgroup$
add a comment |
$begingroup$
The idea behind sandwich/squeeze theorem is intuitive enough. If values of a function $f$ are lying (sandwiched) between those of $g, h$ and values of $g, h$ tend to a common limit then that of $f$ also tend to the same limit.
We have the fundamental inequality $|sin x|leq |x|$ for $0<|x|<pi/2$ which is the same as $$-|x|leq sin xleq |x|$$ for $0<|x|<pi/2$ and since both $-|x|$ and $|x|$ tend to $0$ as $xto 0$ it follows that $lim_{xto 0}sin x=0$.
For $cos x$ the situation is tricky and we need to use a bit of trigonometry. We have $$cos x=1-2sin^2(x/2)$$ and noting the earlier inequality for $sin $ we get $$1-frac{x^2}{2}leq cos xleq 1$$ And by squeeze we get $lim_{xto 0}cos x=1$.
$endgroup$
add a comment |
$begingroup$
The idea behind sandwich/squeeze theorem is intuitive enough. If values of a function $f$ are lying (sandwiched) between those of $g, h$ and values of $g, h$ tend to a common limit then that of $f$ also tend to the same limit.
We have the fundamental inequality $|sin x|leq |x|$ for $0<|x|<pi/2$ which is the same as $$-|x|leq sin xleq |x|$$ for $0<|x|<pi/2$ and since both $-|x|$ and $|x|$ tend to $0$ as $xto 0$ it follows that $lim_{xto 0}sin x=0$.
For $cos x$ the situation is tricky and we need to use a bit of trigonometry. We have $$cos x=1-2sin^2(x/2)$$ and noting the earlier inequality for $sin $ we get $$1-frac{x^2}{2}leq cos xleq 1$$ And by squeeze we get $lim_{xto 0}cos x=1$.
$endgroup$
The idea behind sandwich/squeeze theorem is intuitive enough. If values of a function $f$ are lying (sandwiched) between those of $g, h$ and values of $g, h$ tend to a common limit then that of $f$ also tend to the same limit.
We have the fundamental inequality $|sin x|leq |x|$ for $0<|x|<pi/2$ which is the same as $$-|x|leq sin xleq |x|$$ for $0<|x|<pi/2$ and since both $-|x|$ and $|x|$ tend to $0$ as $xto 0$ it follows that $lim_{xto 0}sin x=0$.
For $cos x$ the situation is tricky and we need to use a bit of trigonometry. We have $$cos x=1-2sin^2(x/2)$$ and noting the earlier inequality for $sin $ we get $$1-frac{x^2}{2}leq cos xleq 1$$ And by squeeze we get $lim_{xto 0}cos x=1$.
answered 21 hours ago
Paramanand SinghParamanand Singh
50.2k556163
50.2k556163
add a comment |
add a comment |
$begingroup$
Edit: I corrected some inaccuracies pointed out in the comments.
I try to explain the logic by solving the exercize. First of all, this result is also known as the two policemen theorem because if two policemen are escorting a person between them, and both officers approch arbitrary close to the cell, then the prisoner must also approch arbitrary close to it (notice that the policemen don't need to go to the cell; this means the functions need not to be defined in the limit point). This summarizes the main idea of the theorem.
Now, we know that when $x$ is sufficiently near to $0$ (i.e. the functions must be defined in a neighbourhood of such a point), $-lvert xrvertle sin xle lvert xrvert$ and this holds for any $x$ sufficiently small (actually for any $lvert xrvert $ in $]0,pi/2[$). Since limits preserve the linear order, we have that if limit of $sin x$ exists, then $$lim_{xrightarrow0} -xle lim_{xrightarrow0} sin x le lim_{xrightarrow0} x.$$
But the first and the last limits are $0$, hence limit of $sin x$ exists and is $0$ because this function is eventually bounded both from above and below by functions converging to the same limit, so $sin x$ can't go away.
Generally speaking, you need two functions/policemen who bound the function of which you want to compute the limit, from above and below respectively, in a neighbourhood of the limit point and they must have the same limit at that point.
$endgroup$
3
$begingroup$
Strictly speaking, the officers don't actually have to enter the cell, they just have to approach arbitrarily close to it, and then the prisoner must also approach arbitrarily close to it. :-)
$endgroup$
– Brian Tung
20 hours ago
1
$begingroup$
There are some inaccuracies in this answer. The inequalities $-x leq sin , xleq x$ can hold only for $x geq 0$ so one can only take right hand limits.
$endgroup$
– Kavi Rama Murthy
20 hours ago
$begingroup$
@BrianTung I was so drunk last night that I don't remember exactly what happened with the officers ;-)
$endgroup$
– LBJFS
16 hours ago
$begingroup$
@KaviRamaMurthy I apologize for the errors and thanks for the mistakes you pointed out.
$endgroup$
– LBJFS
16 hours ago
add a comment |
$begingroup$
Edit: I corrected some inaccuracies pointed out in the comments.
I try to explain the logic by solving the exercize. First of all, this result is also known as the two policemen theorem because if two policemen are escorting a person between them, and both officers approch arbitrary close to the cell, then the prisoner must also approch arbitrary close to it (notice that the policemen don't need to go to the cell; this means the functions need not to be defined in the limit point). This summarizes the main idea of the theorem.
Now, we know that when $x$ is sufficiently near to $0$ (i.e. the functions must be defined in a neighbourhood of such a point), $-lvert xrvertle sin xle lvert xrvert$ and this holds for any $x$ sufficiently small (actually for any $lvert xrvert $ in $]0,pi/2[$). Since limits preserve the linear order, we have that if limit of $sin x$ exists, then $$lim_{xrightarrow0} -xle lim_{xrightarrow0} sin x le lim_{xrightarrow0} x.$$
But the first and the last limits are $0$, hence limit of $sin x$ exists and is $0$ because this function is eventually bounded both from above and below by functions converging to the same limit, so $sin x$ can't go away.
Generally speaking, you need two functions/policemen who bound the function of which you want to compute the limit, from above and below respectively, in a neighbourhood of the limit point and they must have the same limit at that point.
$endgroup$
3
$begingroup$
Strictly speaking, the officers don't actually have to enter the cell, they just have to approach arbitrarily close to it, and then the prisoner must also approach arbitrarily close to it. :-)
$endgroup$
– Brian Tung
20 hours ago
1
$begingroup$
There are some inaccuracies in this answer. The inequalities $-x leq sin , xleq x$ can hold only for $x geq 0$ so one can only take right hand limits.
$endgroup$
– Kavi Rama Murthy
20 hours ago
$begingroup$
@BrianTung I was so drunk last night that I don't remember exactly what happened with the officers ;-)
$endgroup$
– LBJFS
16 hours ago
$begingroup$
@KaviRamaMurthy I apologize for the errors and thanks for the mistakes you pointed out.
$endgroup$
– LBJFS
16 hours ago
add a comment |
$begingroup$
Edit: I corrected some inaccuracies pointed out in the comments.
I try to explain the logic by solving the exercize. First of all, this result is also known as the two policemen theorem because if two policemen are escorting a person between them, and both officers approch arbitrary close to the cell, then the prisoner must also approch arbitrary close to it (notice that the policemen don't need to go to the cell; this means the functions need not to be defined in the limit point). This summarizes the main idea of the theorem.
Now, we know that when $x$ is sufficiently near to $0$ (i.e. the functions must be defined in a neighbourhood of such a point), $-lvert xrvertle sin xle lvert xrvert$ and this holds for any $x$ sufficiently small (actually for any $lvert xrvert $ in $]0,pi/2[$). Since limits preserve the linear order, we have that if limit of $sin x$ exists, then $$lim_{xrightarrow0} -xle lim_{xrightarrow0} sin x le lim_{xrightarrow0} x.$$
But the first and the last limits are $0$, hence limit of $sin x$ exists and is $0$ because this function is eventually bounded both from above and below by functions converging to the same limit, so $sin x$ can't go away.
Generally speaking, you need two functions/policemen who bound the function of which you want to compute the limit, from above and below respectively, in a neighbourhood of the limit point and they must have the same limit at that point.
$endgroup$
Edit: I corrected some inaccuracies pointed out in the comments.
I try to explain the logic by solving the exercize. First of all, this result is also known as the two policemen theorem because if two policemen are escorting a person between them, and both officers approch arbitrary close to the cell, then the prisoner must also approch arbitrary close to it (notice that the policemen don't need to go to the cell; this means the functions need not to be defined in the limit point). This summarizes the main idea of the theorem.
Now, we know that when $x$ is sufficiently near to $0$ (i.e. the functions must be defined in a neighbourhood of such a point), $-lvert xrvertle sin xle lvert xrvert$ and this holds for any $x$ sufficiently small (actually for any $lvert xrvert $ in $]0,pi/2[$). Since limits preserve the linear order, we have that if limit of $sin x$ exists, then $$lim_{xrightarrow0} -xle lim_{xrightarrow0} sin x le lim_{xrightarrow0} x.$$
But the first and the last limits are $0$, hence limit of $sin x$ exists and is $0$ because this function is eventually bounded both from above and below by functions converging to the same limit, so $sin x$ can't go away.
Generally speaking, you need two functions/policemen who bound the function of which you want to compute the limit, from above and below respectively, in a neighbourhood of the limit point and they must have the same limit at that point.
edited 14 hours ago
answered 21 hours ago
LBJFSLBJFS
2407
2407
3
$begingroup$
Strictly speaking, the officers don't actually have to enter the cell, they just have to approach arbitrarily close to it, and then the prisoner must also approach arbitrarily close to it. :-)
$endgroup$
– Brian Tung
20 hours ago
1
$begingroup$
There are some inaccuracies in this answer. The inequalities $-x leq sin , xleq x$ can hold only for $x geq 0$ so one can only take right hand limits.
$endgroup$
– Kavi Rama Murthy
20 hours ago
$begingroup$
@BrianTung I was so drunk last night that I don't remember exactly what happened with the officers ;-)
$endgroup$
– LBJFS
16 hours ago
$begingroup$
@KaviRamaMurthy I apologize for the errors and thanks for the mistakes you pointed out.
$endgroup$
– LBJFS
16 hours ago
add a comment |
3
$begingroup$
Strictly speaking, the officers don't actually have to enter the cell, they just have to approach arbitrarily close to it, and then the prisoner must also approach arbitrarily close to it. :-)
$endgroup$
– Brian Tung
20 hours ago
1
$begingroup$
There are some inaccuracies in this answer. The inequalities $-x leq sin , xleq x$ can hold only for $x geq 0$ so one can only take right hand limits.
$endgroup$
– Kavi Rama Murthy
20 hours ago
$begingroup$
@BrianTung I was so drunk last night that I don't remember exactly what happened with the officers ;-)
$endgroup$
– LBJFS
16 hours ago
$begingroup$
@KaviRamaMurthy I apologize for the errors and thanks for the mistakes you pointed out.
$endgroup$
– LBJFS
16 hours ago
3
3
$begingroup$
Strictly speaking, the officers don't actually have to enter the cell, they just have to approach arbitrarily close to it, and then the prisoner must also approach arbitrarily close to it. :-)
$endgroup$
– Brian Tung
20 hours ago
$begingroup$
Strictly speaking, the officers don't actually have to enter the cell, they just have to approach arbitrarily close to it, and then the prisoner must also approach arbitrarily close to it. :-)
$endgroup$
– Brian Tung
20 hours ago
1
1
$begingroup$
There are some inaccuracies in this answer. The inequalities $-x leq sin , xleq x$ can hold only for $x geq 0$ so one can only take right hand limits.
$endgroup$
– Kavi Rama Murthy
20 hours ago
$begingroup$
There are some inaccuracies in this answer. The inequalities $-x leq sin , xleq x$ can hold only for $x geq 0$ so one can only take right hand limits.
$endgroup$
– Kavi Rama Murthy
20 hours ago
$begingroup$
@BrianTung I was so drunk last night that I don't remember exactly what happened with the officers ;-)
$endgroup$
– LBJFS
16 hours ago
$begingroup$
@BrianTung I was so drunk last night that I don't remember exactly what happened with the officers ;-)
$endgroup$
– LBJFS
16 hours ago
$begingroup$
@KaviRamaMurthy I apologize for the errors and thanks for the mistakes you pointed out.
$endgroup$
– LBJFS
16 hours ago
$begingroup$
@KaviRamaMurthy I apologize for the errors and thanks for the mistakes you pointed out.
$endgroup$
– LBJFS
16 hours ago
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3119710%2fhow-does-the-squeeze-theorem-imply-that-lim-theta-to-0-sin-theta-0-and-l%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
1
$begingroup$
Are you already equipped with the series representation of $sin theta$ and $cos theta$ and their approximation by Taylor polynomials? Because first you need an appropriate approximation of these functions near $0$ and then we could check how the sandwich trick works.
$endgroup$
– trancelocation
21 hours ago