\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  e=\frac{c}{c_0+c}e_\text{max}.
\end{equation*}
\end{document}