9. Find the radius of convergence of each of the following power series: \text { (a) } \sum n^{3} z^{n}, \text { (b) } \sum \frac{2^{n}}{n !} z^{n} \text {

(c) } \sum \frac{2^{n}}{n^{2}} z^{n} \text {, } \text { (d) } \sum \frac{n^{3}}{3^{n}} z^{n}