(a) For a, d ∈ R, find the generating function for the sequence a, a + d, a + 2d, a + 3d, ....
(b) For n ∈ Z+, use the result from part (a) to find a formula for the sum of the first n terms of the arithmetic progression a, a -f- d, a -f- 2d, a -f- 3d, ....