Chapter 31 - Electromagnetic Oscillations and Alternating Current - Problems - Page 939: 69a

$\phi = -0.405~rad$

We can find the phase constant: $tan~\phi = \frac{X_L-X_C}{R}$ $tan~\phi = \frac{\omega_d~L-1/\omega_d~C}{R}$ $tan~\phi = \frac{2\pi~f~L-1/2\pi~f~C}{R}$ $tan~\phi = \frac{(2\pi)~(3000~Hz)~(850\times 10^{-6}~H)-1/(2\pi)~(3000~Hz)~(1.60\times 10^{-6}~F)}{40.0~\Omega}$ $tan~\phi = -0.42838$ $\phi = tan^{-1}~(-0.42838)$ $\phi = -0.405~rad$