6. Find a closed form for the generating function for the sequence {an}, where a) an = -1 for all n = 0, 1, 2,.... b) an = 2" for n = 1, 2, 3, 4, ... and ao = 0. c) an=n-1 for n = 0, 1, 2, .... d) an = 1/(n+1)! 1/(n + 1)! for n = 0, 1, 2,.... e) an = n (2) for n = 0, 1, 2,....