a. \(R_b=p\dfrac{l}{S}=1,1\cdot10^{-6}\dfrac{10}{0,55\cdot10^{-6}}=20\Omega\)
b. \(MCD:R_dntR_b\)
\(\Rightarrow\left\{{}\begin{matrix}U_b=U-U_d=40-30=10V\\I_b=I_d=I=\dfrac{P_d}{U_d}=\dfrac{30}{30}=1A\end{matrix}\right.\)
\(\Rightarrow R_b'=\dfrac{U_b}{I_b}=\dfrac{10}{1}=10\Omega\)