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In one big align environment, I want to put some cases environments. I would like the alignment of the cases environments to be the same. For example, if I write:

\begin{align}
f(x) &= 
  \begin{cases}
    x & \text{if } x\geq 0,\\
    -x & \text{if } x <0,
  \end{cases}\\
g(x) &=
  \begin{cases}
    \sqrt{x} & \text{if } x \geq 0,\\
    -\sqrt{-x} & \text{if } x < 0.
  \end{cases}
\end{align}

then the equation looks ugly because the two cases environment are aligned at different points. Is there a way to force the alignment point of the two environment to be the same?

share|improve this question
    
See also Aligning the right parts in multiple \begin{cases} (possible duplicate), but here you have the better answers. –  Hendrik Vogt Feb 3 '13 at 8:09
    
I searched for it, but I did not find anything. Thanks! –  antonio Feb 4 '13 at 17:12

3 Answers 3

You could also use a \hphantom{} to add the appropriate amount of horizontal space:

enter image description here

Code:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
f(x) &= 
  \begin{cases}
    x\hphantom{-\sqrt{-}} & \text{if } x\geq 0,\\
    -x & \text{if } x <0,
  \end{cases}\\
g(x) &=
  \begin{cases}
    \sqrt{x} & \text{if } x \geq 0,\\
    -\sqrt{-x} & \text{if } x < 0.
  \end{cases}
\end{align}
\end{document}
share|improve this answer
    
I was hoping in something that does not require some sort of manual spacing, that I could also use in more complicated examples. Thank you anyway! –  antonio Feb 4 '13 at 17:16

You can enclose one of the entries in the first cases environment in a box whose width is that of the longest of the second cases environment.

\newlength{\temp}
\settowidth{\temp}{$-\sqrt{-x}$}
\begin{align}
f(x) &= 
  \begin{cases}
    \makebox[\temp][l]{$x$} & \text{if } x\geq 0,\\
    -x & \text{if } x <0,
  \end{cases}\\
g(x) &=
  \begin{cases}
    \sqrt{x} & \text{if } x \geq 0,\\
    -\sqrt{-x} & \text{if } x < 0.
  \end{cases}
\end{align}

enter image description here

share|improve this answer

Use a combination of hphantom and mathrlap provided by the package mathtools. Put the longest expression in hphantom and wrap mathrlap around the expression that hphantom is placed after.

In your example, the longest expression is -\sqrt{-x}. To place hphantom where x is, use

\mathrlap{x}\hphantom{-\sqrt{-x}}

For example:

\documentclass{article}
\usepackage{amsmath,mathtools}
\begin{document}
\begin{align}
  f(x) &= \begin{cases}
    \mathrlap{x}\hphantom{-\sqrt{-x}} & \text{if } x\geq 0,\\
    -x & \text{if } x <0,
  \end{cases}\\
  g(x) &= \begin{cases}
    \sqrt{x} & \text{if } x \geq 0,\\
    -\sqrt{-x} & \text{if } x < 0.
  \end{cases}
\end{align}
\end{document}
share|improve this answer
    
Please post a full MWE, not code snippets. –  Mico Apr 21 at 10:53

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