\documentclass[convert={density=500}, border=2pt, varwidth=2in]{standalone}

\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}

\begin{align*}
\rho_{X,Y} &= \frac{\mathrm{cov}\left(X, Y\right)}{\sigma_X \sigma_Y}
\end{align*}

\end{document}