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\begin{equation*}
  \frac{d G_{un}}{dP} = \frac{d}{d P} \left( -\frac{C}{1+PC} \right)
  = \frac{C}{(1 + PC)^2} C = {\color{blue} -}T \frac{G_{un}}{P}
\end{equation*}
\end{document}