Htydjtk

I would like to align the + and the if's below eachother but for some reason the ifs all get aligned to the right even though I used the &. I don't understand why this is happening and I would like to know how to fix it such that the ifs come under each other.

documentclass[12pt]{article}

usepackage{amsmath}



begin{document}



begin{equation*}

left{begin{align*}

        f(x) quad &+ quad g(x) & text{ if } f(x)geq 0 text{, } g(x)     geq 0 \

        0 quad &+ quad 0 & text{ if } f(x)geq 0 text{, } g(x) < -f(x) \

        f(x)-(-g(x)) quad &+ quad 0 & text{ if } f(x)geq 0 text{, } -f(x) leq g(x) < 0 \

        0 quad &+ quad 0 & text{ if } f(x)<0 text{, } g(x)< 0 \

        end{align*}

        right.

end{equation*}

The first, third, fifth, ... & left-align the following expressions, while the second, fourth, sixth, ... & right-align the following expressions. Your solution here: Use && instead of & at the second appearance in each line:

documentclass[12pt]{article}

usepackage{amsmath}

begin{document}

begin{equation*}

left{begin{aligned}

        f(x) quad &+ quad g(x) && text{ if } f(x)geq 0 text{, } g(x)     geq 0 \

        0 quad &+ quad 0 && text{ if } f(x)geq 0 text{, } g(x) < -f(x) \

        f(x)-(-g(x)) quad &+ quad 0 && text{ if } f(x)geq 0 text{, } -f(x) leq g(x) < 0 \

        0 quad &+ quad 0 && text{ if } f(x)<0 text{, } g(x)< 0 \

end{aligned}right.

end{equation*}

end{document}

I see no reason for aligning at the + sign, but perhaps you have.

documentclass[12pt]{article}

usepackage{amsmath}



begin{document}



Alignment at $+$

begin{equation*}

begin{cases}

begin{alignedat}{2}

f(x)         &+ g(x) &qquad& text{if $f(x)geq 0$, $g(x)geq 0$} \

0            &+ 0    &qquad& text{if $f(x)geq 0$, $g(x) < -f(x)$} \

f(x)-(-g(x)) &+ 0    &qquad& text{if $f(x)geq 0$, $-f(x) leq g(x) < 0$} \

0            &+ 0    &qquad& text{if $f(x)<0$, $g(x)< 0$}

end{alignedat}

end{cases}

end{equation*}



Better with no artificial alignment?

begin{equation*}

begin{cases}

f(x) + g(x)      & text{if $f(x)geq 0$, $g(x)geq 0$} \

0 + 0            & text{if $f(x)geq 0$, $g(x) < -f(x)$} \

f(x)-(-g(x)) + 0 & text{if $f(x)geq 0$, $-f(x) leq g(x) < 0$} \

0 + 0            & text{if $f(x)<0$, $g(x)< 0$}

end{cases}

end{equation*}



end{document}

The top display could have been simply managed with aligned, but because of the alignment at +, I think it's better to have more space between the two parts. Try with aligned and && instead of &qquad&.

That's the normal behaviour if you specify alignment points with an ampersand: it is implicitly added at the end of each line. Furthermore, n columns of alignment require 2n–1 ampersands: one ampersang to introduce each new column but the first, and one ampersand to set the alignment point inside that column.

I also propose a simpler code, with the empheq package (wich loads amthtools, which loads amsmath). I suppose this is what you want:

documentclass[12pt]{article}

usepackage{empheq}



begin{document}



    begin{empheq}[left=empheqlbrace]{align*}

        f(x) quad &+ quad g(x) &text{ if }& f(x)geq 0, g(x) geq 0 \

        0 quad &+ quad 0 & text{ if } & f(x)geq 0 , g(x) < -f(x) \

        f(x)-(-g(x)) quad &+ quad 0 & text{ if } & f(x)geq 0 , -f(x) leq g(x) < 0 \

        0 quad &+ quad 0 & text{ if } & f(x)<0 , g(x)< 0 \

   end{empheq}



end{document} 

with use of array:

documentclass[12pt]{article}

usepackage{array}

usepackage{amsmath}



begin{document}

[setlengtharraycolsep{2pt}

left{begin{array}{rcl @{qquad}r l}

        f(x) & + & g(x) & text{if }    & f(x)geq 0, g(x) geq 0 \

          0  & + & 0    & text{if }    & f(x)geq 0, g(x) < -f(x) \

f(x)-(-g(x)) & + & 0    & text{if }    & f(x)geq 0, -f(x) leq g(x) < 0 \

        0    & + & 0    & text{if }    & f(x)<0 , g(x)< 0 \

   end{array}right.

]



end{document}