For the circuit shown in Figure P5.21, assume that switch $S_1$ is always open; switch $S_2$ has been closed for a long time, and opens at $f=0$.
a. Find the capacitor voltage, $v_C(t)$, at $t=0^{+}$.
b. Find the time constant, $\tau$, for $t \geq 0$.
c. Find an expression for $v_c(t)$ and sketch the function.
d. Find $v_c(t)$ for each of the following values of $t$ : $0, \tau, 2 \pi, 5 \tau, 10 \tau$.