9. Given X ~ Geometric(p) and X1, X2, ... independent with the distribution of X, observe that Y = X1+ X2+ . .. + Xr, represents the number of tosses of a biased (p) coin required to obtain r heads. a) Derive the pof of Y. b) Find P(Y = k), the coefficient of the in the power series expansion of the pgf. Y is said to have the negative binomial distribution. Can you see why