The switch in the circuit shown in Fig. $7\mathrm{.}27$ has been open for a long time. The initial charge on the capacitor is zero. At $t=0,$ the switch is crosed. Find the expression for

a$)i(t)$ for $t{0}^{+}$and

b$)v(t)$ when $t{0}^{+}.$

Figure $7\mathrm{.}27$ A