\documentclass[12pt]{article}
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\begin{document}
\begin{equation*}
  G_{ed}(s) = -
  \frac{d_c(s) n_p(s)}{(s+a) d_c(s) d'_p(s) + (s+a) n'_c(s) n_p(s)}.
\end{equation*}
\end{document}