\documentclass[convert={density=500}, border=2pt, varwidth=3in]{standalone}

\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}

\begin{align*}
r_{X, Y} &= \frac{
\sum_{i=1}^n(x_i - \overline{x})(y_i - \overline{y})
}{
\sqrt{
\sum_{i=1}^n(x_i-\overline{x})^2
}
\sqrt{
\sum_{i=1}^n(y_i-\overline{y})^2
}
}
\end{align*}

\end{document}