Math Is Fun Forum

Hi everyone. I'm stumped with one of my real analysis homework problems:

Let ∑an be a series and Sn its sequence of partial sums. Suppose ∑an*x^n converges when |x|<1 to f(x). Show that ∑Sn*x^n = f(x)/(1-x) for |x|<1.

Thanks very much!