The relations considered in this problem are used on many occasions throughout the book. (a) Prove the validity of the following expression: N-1 n=0 (d) Evaluate N, 1-a 1-a This is often referred to as the finite sum formula. (b) Show that if a < 1, then assuming that a < 1. a = 1 for any complex number a # 1* n=0 This is often referred to as the infinite sum formula. (c) Show also if a < 1, then n=0 = na" = 1 (1a) ", n=k