After traveling steadily at 400 meters above a shipwrecked hull, a submerged vessel starts to descend when its ground distance from the hull is 7 kilometers. What is the angle of depression for this part of the travel ?

Let x have probability generating function gx (s) and let un generating function u(s) of the sequence uo, u1, satisfies p(x > n). show that the (1- s)u(s) = 1 - gx(s), whenever the series defining these generating functions converge.