\documentclass[12pt]{article}
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\begin{document}
\begin{equation*}
  H_i = \frac{C_i}{1 + C_i P_i} - mg \frac{C_i P_i}{1 + C_i P_i}
      = \frac{C_i (1 - m g P_i)}{1 + C_i P_i}.
\end{equation*}
\end{document}