9. Suppose that independent Bernoulli trials with parameter p are performed successively. Let N be the number of trials needed to get x successes, and X be the number of successes in the first n trials. Show that P (N = n) = x n P (X = x). Remark: By this relation, in coin tossing, for example, we can state that the probability of getting a fifth head on the seventh toss is 5/7 of the probability of five heads in seven tosses.