1. The problem statement, all variables and given/known data Find the power series representation of f(x)= 1/((4+x)^2). Use this representation to determine the sum of the series: sum [1, infinity) of n*(3/8)^n. I attached a screenshot of the problem for just in case. 2. Relevant equations 3. The attempt at a solution What I did first was find the power series representation of f(x)= 1/(4+x). I got sum [0, infinity) of ((-1)^n))(x^n)/(4^(n+1)). and then I got the derivative of that which made it: sum [1, infinity) of ((-1)^n))(n)(x^(n-1))/(4^(n+1)). And so that is where I am lost. I don't know how to use that representation to find the sum of the series "n*(3/8)^n".