\documentclass[preview]{standalone}
\usepackage[english]{babel}
\usepackage{amsmath}
\usepackage{amssymb}
\begin{document}
\begin{align*}
\frac{d}{dx}f(x)g(x)= f(x)\frac{d}{dx}g(x) + g(x)\frac{d}{dx}f(x)
\end{align*}
\end{document}