s=1/a+2/a^2+3/a^3+……+n/a^n as=1+2/a+.+n/a^(n-1) (a-1)s=1+1/a+1/a^2.+1/a^(n-1)-n/a^n =[1-(1/a)^n]/(1-1/a)-n/a^n s=[a(a^n-1)]/[(a-1)^2]-n/[a^n*(a-1)]