
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
%\usepackage{lmodern}
\pagestyle{empty}
\begin{document}
\[
\left \{
\begin{array}{c @{=} c}
    x & \sin a \cos b \\
    y & \sin a \sin b
\end{array}
\right.
\]
\end{document}
