Đặt điện áp $u={{U}_{0}}\text{cos}\omega \text{t}$ (...

$$Ta có $\Delta \varphi ={{\varphi }_{AB}}-{{\varphi }_{MB}}\Rightarrow \tan \Delta \varphi =\frac{\tan {{\varphi }_{AB}}-\tan {{\varphi }_{MB}}}{1+\tan {{\varphi }_{AB}}\tan {{\varphi }_{MB}}}=\frac{-{{Z}_{C}}\left( \frac{1}{4{{R}_{2}}}-\frac{1}{{{R}_{2}}} \right)}{1+\frac{Z_{C}^{2}}{4R_{2}^{2}}}=\frac{\frac{3{{Z}_{C}}}{4{{R}_{2}}}}{1+\frac{Z_{C}^{2}}{4R_{2}^{2}}}=\frac{\frac{3}{4{{R}_{2}}}}{\frac{1}{{{Z}_{C}}}+\frac{{{Z}_{C}}}{4{{R}_{2}}}}{{\left( \tan \Delta \varphi \right)}_{\max }}\Leftrightarrow \frac{1}{{{Z}_{C}}}+\frac{{{Z}_{C}}}{4{{R}_{2}}}\min \frac{1}{{{Z}_{C}}}+\frac{{{Z}_{C}}}{4{{R}_{2}}}\ge \frac{1}{{{R}_{2}}}''=''\frac{1}{{{Z}_{C}}}=\frac{{{Z}_{C}}}{4{{R}_{2}}}\Leftrightarrow {{Z}_{C}}=2{{R}_{2}}=20\Omega $