\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
u(t) = \sum_{k=1}^{\infty} a_k \sin (k \omega t) + b_k \cos (k\omega t).
\end{equation*}
\end{document}