How do I box an equation array?

Question

I want to have an equation array (the environment eqnarray) contained in a box. The only way I know is to use the \boxed command. This works for

\begin{eqnarray}
\boxed{1+1}&=&2
\end{eqnarray}

but fails for

\begin{eqnarray}
\boxed{1+1&=&2}
\end{eqnarray}

How should I box the whole thing?

Answer 1

eqnarray is an environment from the LaTeX kernel which does its job, but there is better on the market. You use the \boxed command, which means that you are already loading amsmath, so you should use its display alignment environments. This being said, it's true that \boxed doesn't work across cells. The package mathtools (an extension to amsmath) provides a macro \Aboxed which does.

\documentclass{article}

\usepackage{mathtools} % loads amsmath

\begin{document}

\begin{align}
\Aboxed{ a&=b } \\
c&= d
\end{align}

\end{document}

Answer 2

If you want to put multiple aligned equations in a box I'd recommend the empheq package. For example:

\documentclass{article}
\usepackage{amsmath,empheq}
\begin{document}

\begin{empheq}[box=\fbox]{align*}
a &= b \\
c &= d
\end{empheq}

\end{document}