
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{mathtools}
\usepackage{lmodern}
\pagestyle{empty}
\begin{document}
\begin{align*}
a &= \left( \frac{1}{2} + b + c 
     + \ldots \right. \\
  & \qquad \left. 
     \vphantom{\frac{1}{2}} 
     + \ldots + x + y + z \right)^2
\end{align*}
\end{document}
