I am looking for the general arithmetic-geometric series of n terms in the form of: a = (a+d)r + (a + 2d)r^2 + ...+[a + (n-1)d]r^n-1

where the first factors of each term form an arithmetic series and the second factors from a geometric series. I am looking for a website that shows this step by step so I can be able to explain this in my education class, I am in calculus and we have not covered this material as of yet. I looked in my calculus text and the method to derive this formula is not listed. Can someone point me in the right direction.