\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\text{var} = \frac{N}{N-1} \left[ \left\langle x^2 \right\rangle - \left\langle x \right\rangle^2\right]
\end{equation*}
\end{document}