Let $f(z)$ be analytic in a region $R$ bounded by two concentric circles $C_1$ and $C_2$ and on the boundary [Fig. 5-11]. Prove that if $z_0$ is any point in $R$, then
$$
f\left(z_0\right)=\frac{1}{2 \pi i} \oint_{C_1} \frac{f(z)}{z-z_0} d z-\frac{1}{2 \pi i} \oint_{C_2} \frac{f(z)}{z-z_0} d z
$$