It converges. Because: lim(Tn+1 / Tn) -> a < 1. To sum, you got to factorize like a^n*b^2 + 2*b*c*n*a^n+c^2*n^2*a^n Sum(a^n*b^2)=b^2*a/(1-a) Sum(2*b*c*n*a^n) = 2*b*c*Sum(n*a^n) Sum(c^2*n^2*a^n) = c^2*Sum(n^2*a^n) You now got to solve Sum(n*a^n) and Sum(n^2*a^n) separately. Sum(n*a^n)*a - Sum(n*a^n)= a*(2a-1)/(1-a), so: Sum(n*a^n) = a*(1-2a)/(1-a)^2 Use same logic to work out Sum(n^2*a^n) and then sum.