1. The problem statement, all variables and given/known data Find the interval of convergence of f'(x) f(x) Sum from n=1 to infinity [(x-5)^n*(-1)^n]/[n5^n] 2. Relevant equations 3. The attempt at a solution My problem is I am unsure how to take the derivative with the n's and x's should I treat n as a constant? After that, I think I can get the interval of convergence.