\(U=60V\Rightarrow U_0=U\sqrt{2}=60\sqrt{2}V\)
\(RLC: i_1=I_0.cos(100 \pi t +\frac{\pi}{4})(A)\)
\(RL: i_2=I_0.cos(100 \pi t -\frac{\pi}{12})(A)\)
\(\Rightarrow u=60\sqrt{2}.cos(100 \pi t +\varphi _u)(V)\)
\(\left\{\begin{matrix} \varphi _{i_1}=\varphi _u-\varphi _1\\ \varphi _{i_2}=\varphi _u-\varphi _2 \end{matrix}\right.\Rightarrow \varphi _u= \frac{\varphi _{i_1}+\varphi _1+\varphi _{i_2}+\varphi _2}{2}\)
\(\varphi _u=\frac{\frac{\pi}{6}+\varphi _1+\varphi _2}{2} \ \ (1)\)
\(I_0=\frac{U}{Z_1}=\frac{U}{Z_2}\Rightarrow Z_1=Z_2\Rightarrow R^2+Z_{LC}^2=R^2+Z_L^2\)
\(\Rightarrow Z^2_{LC}=Z^2_L\Rightarrow \left | Z_L-Z_C \right |=Z_L\)
\(\Rightarrow Z_{L}-Z_C =-Z_L\Rightarrow \varphi _1=-\varphi _2 \ \ \ (2)\)
Từ (1) (2) \(\Rightarrow \varphi _{u}=\frac{\pi}{12}\)
⇒ Chọn C