Find a generating function A(x) that generates the sum of the first n integers, i.e., A(x) = 0 + 1x + 3x^2 + + (0 + 1 + 2 + + n)x^n+ . . . , express it as a rational function and then obtain a formula for [x^n]A(x). You should get n(n+1)/ 2 .