\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsfonts}

\thispagestyle{empty}
\begin{document}
\begin{equation}
  \rho(R)=\frac{\rho_0}{\frac{R}{R_s}(1+\frac{R}{R_s})^2}\notag
\end{equation}
\end{document}