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\begin{equation*}
  k_p = \frac{R_1 C_1 + R_2 C_2}{R_1 C_2}, \qquad 
  T_i = R_1 C_1 + R_2 C_2, \qquad 
  T_d = \frac{R_1 R_2 C_1 C_2}{R_1 C_1 + R_2 C_2}.
\end{equation*}
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