Curly braces in arrays

Question

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.

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

Answer 1

The \left. and \right\} instructions have to be placed outside the array environment.

\documentclass{article}
\begin{document}

\[ 
\left. \begin{array}{r} 
\frac{1}{2}( \pi_b+ \pi_a) \pi_b\\[1ex]
{}+\frac{1}{2} \pi_b(1-\pi_a-\pi_b)
\end{array} \right\} 
=\pi_b/2
\]

\end{document}