Answer
a. 1.9 mA.
b. 3.9 mA.
Work Step by Step
a. We are asked to make a very rough approximation. See Figure 29-31. We see that half the time, the current is zero. A rough approximation says that the average current is half of the full rms current.
$$I_{average}=\frac{1}{2}\frac{V_{rms}}{R}=\frac{1}{2}\frac{120V}{31k\Omega}=1.9mA$$
b. We are asked to make a very rough approximation. See Figure 29-32. The sine wave is modified so that the current is positive all of the time. A rough approximation says that the average current is equal to the full rms current.
$$I_{average}=\frac{V_{rms}}{R}=\frac{120V}{31k\Omega}=3.9mA$$