43. An urn containsI mage balls, of which n are red and m are black.

They are withdrawn from the urn, one at a time and without replacement. Let X be the number of red balls removed before the first black ball is chosen. We are interested in determining . To obtain this quantity, number the red balls from 1 to n. Now define the random variablesI mage, by Image

(a) Express X in terms of the Image.

(b) Find .