Lời giải:Cường độ dòng điện trong hai trường hợp bằng nhau :
\(\begin{array}{l}
\Rightarrow cos{\varphi _1} = cos{\varphi _2}\\
\Leftrightarrow \cos \left( {{\varphi _U} - \frac{\pi }{7}} \right) = \cos \left( {{\varphi _U} - \frac{{10\pi }}{{21}}} \right)\\
\Leftrightarrow {\varphi _U} - \frac{\pi }{7} = - {\varphi _U} + \frac{{10\pi }}{{21}}\\
\Leftrightarrow {\varphi _U} = \frac{{13\pi }}{{42}}
\end{array}\)
\(\Rightarrow tan \varphi_1 = \frac{Z_L}{R}=\frac{\sqrt{3}}{3}\Leftrightarrow Z_L=\frac{10\sqrt{3}}{3} \Omega\)
=> \(U= I_1.Z_{RL}=\frac{80\sqrt{3}}{3}\)
=> \(P=\frac{{{U}^{2}}}{R}=\frac{640}{3}=213,33W\).
