\documentclass{article}



\begin{document}

\[ \left| \sum_{i=1}^na_i\bar b_i\right|^2
   \leq 
   \left( \sum_{i=1}^n |a_i|^2 \right) 
   \left( \sum_{i=1}^n |b_i|^2 \right)  \]

\end{document}
