Điện áp hai đầu tụ điện: \(U_c = \frac{U.Z_C}{\sqrt{R^2+(Z_L-Z_C)r}} = \frac{U.\frac{1}{\omega C}}{\sqrt{R^2+(Z_L-Z_C)^2}}\)
\(\Rightarrow \frac{U}{\sqrt{\frac{R^2}{\omega ^2 C^2}+\left ( \frac{L}{C} - \frac{1}{\omega ^2 C^2} \right )^2}} = \frac{U}{\sqrt{R^2\left (\frac{1}{\omega ^2 C^2} \right )+\left ( \frac{L}{C} \right )^2 -2\frac{L}{C}\left ( \frac{1}{\omega ^2 C^2} \right ) + \left (\frac{1}{\omega ^2 C^2} \right )^2}}\)
Đặt \(y = R^2\left (\frac{1}{\omega ^2 C^2} \right )+\left ( \frac{L}{C} \right )^2 -2\frac{L}{C}\left ( \frac{1}{\omega ^2 C^2} \right ) + \left (\frac{1}{\omega ^2 C^2} \right )^2\)
\(U_{C\ max} \Leftrightarrow y_{min} \Leftrightarrow \frac{1}{\omega ^2 C^2} = - \frac{\left ( R^2 - 2 \frac{L}{C} \right )}{2} \Rightarrow \omega = \sqrt{\frac{2}{2LC - R^2C^2}}\)