# Curly braces in arrays

How is it possible to make a big right curly brace and have something like this in an equation array block?

My code doesn't work, can anyone find the problem? The left curly brace at the end of the array is small, not big.

$\begin{array}{l} (\frac{1}{2} \pi_b+ \frac{1}{2}\pi_a) \pi_b+\\ \frac{1}{2} \pi_b(1-\pi_a-\pi_b) \end{array} \right. \left\} =\pi_b/2$

• mathtools has an rcases env for this purpose – daleif Nov 19 '15 at 9:20
• place \left. before the array and \right\} after it – cgnieder Nov 19 '15 at 9:21

The \left. and \right\} instructions have to be placed outside the array environment.
\documentclass{article}
$\left. \begin{array}{r} \frac{1}{2}( \pi_b+ \pi_a) \pi_b\\[1ex] {}+\frac{1}{2} \pi_b(1-\pi_a-\pi_b) \end{array} \right\} =\pi_b/2$