# [Tex/LaTex] How to put a side brace around several lines in the align* environment

align

I would like to put a side brace around several lines in the align* environment, as shown in the comments in the code below:

\begin{align*}
left side & = right side \\
left side & = right side \\  %   }
left side & = right side \\  %   }-  these three lines are important
left side & = right side \\  %   }
left side & = right side \\
\end{align*}


#### Best Answer

You don't need any additional packages to do this. Here's a way using a regular array:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\noindent
\begin{minipage}{.5\textwidth}
\begin{align*}
f(x) & = a \\
g(x) & = ax + b \\
h(x) & = ax^2 + bx + c \\
i(x) & = ax^3 + bx^2 + cx + d \\
j(x) & = ax^4 + bx^3 + cx^2 + dx + e
\end{align*}
\end{minipage}%
\begin{minipage}{.5\textwidth}
$\setlength{\jot}{4.5pt} \begin{array}{r@{\;}l} f(x) & = a \\[\jot] g(x) & = ax + b \\[\jot] h(x) & = ax^2 + bx + c \hspace*{3em} \smash{\left.\begin{array}{@{}c@{}}\\[\jot]\\[\jot]\\[\jot]\end{array}\right\}} \\[\jot] i(x) & = ax^3 + bx^2 + cx + d \\[\jot] j(x) & = ax^4 + bx^3 + cx^2 + dx + e \end{array}$
\end{minipage}%
\end{document}


On the left is the regular align* environment (from amsmath), while the right should an array. A zero-height 3-row array with only a right \} delimiter is added to the middle equation to group the output.

Another option, this time using blkarray:

\documentclass{article}
\usepackage{blkarray}% http://ctan.org/pkg/blkarray
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\noindent
\begin{minipage}{.5\textwidth}
\begin{align*}
f(x) & = a \\
g(x) & = ax + b \\
h(x) & = ax^2 + bx + c \\
i(x) & = ax^3 + bx^2 + cx + d \\
j(x) & = ax^4 + bx^3 + cx^2 + dx + e
\end{align*}
\end{minipage}%
\begin{minipage}{.5\textwidth}
$\def\arraystretch{1.1} \begin{blockarray}{r@{\;}l} f(x) & = a \\[\jot] \begin{block}{r@{\;}l\}} g(x) & = ax + b \\[\jot] h(x) & = ax^2 + bx + c \\[\jot] i(x) & = ax^3 + bx^2 + cx + d \\[\jot] \end{block} j(x) & = ax^4 + bx^3 + cx^2 + dx + e \end{blockarray}$
\end{minipage}%
\end{document}


You may have to play around with the vertical displacement, compared to the align* environment.