P(R) =\( \frac{R·U^2}{ (Ri + R)^2} \)
\( P^{\prime}(R)=\frac{U^{2} \cdot\left(R_{i}+R\right)^{2}-R U^{2} \cdot 2 \cdot\left(R_{i}+R\right)}{\left(R_{i}+R\right)^{4}} \)
\( P^{\prime}(R)=\frac{U^{2} \cdot\left(R_{i}+R\right)-2 R U^{2}}{\left(R_{i}+R\right)^{3}} \)
\( \frac{U^{2} \cdot\left(R_{i}+R\right)-2 R U^{2}}{\left(R_{i}+R\right)^{3}}=0 \) wobei \( \left(R_{i}+R\right) \neq 0 \)
\( U^{2} \cdot\left(R_{i}+R\right)-2 R U^{2}=0 \mid: U^{2} \)
\( R_{i}+R=2 R \)
\( R_{i}=R \)