\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
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\begin{document}
\begin{equation*}
  \frac{1}{2} \Bigl(\frac{d\varphi}{dt} \Bigr)^2 - \varphi + \varphi_0
  - a\cos{\varphi} - \sqrt{a^2-1}=0.
\end{equation*}
\end{document}