\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\sigma=\sqrt { \frac{1}{N} \sum_i x_i^2 - \frac{1}{N^2}(\sum_i x_i)^2 }
\end{equation*}
\end{document}