Cảm kháng: 

\(Z_L=\omega L=100\pi.\frac{3}{10\pi }=30\Omega\)

Dung kháng: 

\(Z_C=\frac{1}{\omega C}=\frac{1}{100\pi.\frac{10^{-3}}{7\pi }}=70\Omega\)

Tổng trở: 

\(\begin{array}{l}
Z = \sqrt {{R^2} + {{({Z_L} - {Z_C})}^2}} \\
 = \sqrt {{R^2} + {Z_L}2} \\
 = \sqrt {{{40}^2} + {{30}^2}}  = 50\Omega 
\end{array}\)

Suy ra: 

\(\begin{array}{l} {I_0} = \frac{{{U_{AF}}}}{{{Z_{AF}}}} = \frac{{120}}{{50}} = 2,4(A)\\ tan{\varphi _{AF}} = \frac{{{Z_L}}}{R} = \frac{{20}}{{40}} = 0,75\\ \Rightarrow {\varphi _{AF}} = \frac{{37\pi }}{{180}}(rad)\\ {\varphi _u} = {\varphi _i} + {\varphi _{AF}} = 0\\ \to {\varphi _i} = - \frac{{37\pi }}{{180}}(rad) \end{array}\)

Vậy, \(i=2,4cos(100\pi t-\frac{37\pi }{180})(A)\)