# Using align inside cases

I was trying to achieve an alignment inside a cases environment. Here is what I was trying to achieve:

I actually was able to achieve the above using the following MathJax code:

f(x,y) = \begin{cases} \begin{align*} x&=1, & y&=1 \\ x&=10, & y&=20 \end{align*} \end{cases}


I realize that $$ is not proper LaTeX syntax, but I'm not sure what to substitute it with. Now, when I try to put this inside a LaTeX document, it gives me a few errors and doesn't line everything up properly into two columns (there is only one column). \documentclass{article} \usepackage{amsmath} \begin{document}$$
f(x,y) =
\begin{cases}
\begin{align*}
x&=1, & y&=1 \\
x&=10, & y&=20
\end{align*}
\end{cases}
\end{document}  Here is something else I tried, which reduced the graphical errors, but still puts it inside only one column: \documentclass{article} \usepackage{amsmath} \begin{document} \begin{align*} f(x,y) = \begin{cases} u &= \ln x & v' &= 1 \\ u' &= \frac{1}{x} & v &= x \\ \end{cases} \end{align*} \end{document}  I would also like to control the spacing with the alignat environment, but I need to find out how to do this first before I can start thinking about that :) • texdoc amsmath ? align is a display environment (you can use aligned for subterms. Sep 24 '17 at 16:10 • the latex equivalent to ..$$ (which is also the mathjax default) is $..$ Sep 24 '17 at 16:12 • Don't use$$ ... . See the answer to this question. Sep 24 '17 at 16:15
• MathJax is not LaTeX; it (wrongly) allows constructions that are illegal as LaTeX code and this is an example. Sep 24 '17 at 16:29

The simplest is to use empheq + alignat*. Needless to load amsmath, since empheq loads mathtools, which does it for you.

\documentclass{article}%
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{empheq}

\begin{document}

\begin{empheq}[left={f(x, y)=\empheqlbrace}]{alignat*=2}
x &=1, &\quad y &=1 \\
x &=10, & y &=20
\end{empheq}

\end{document}


align is for complete display math, aligned is the version for subterms.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

f(x,y) = \left\{ \begin{aligned} x&=1, & y&=1 \\ x&=10, & y&=20 \end{aligned} \right.

\end{document}