求和:1+3a+5a^2+…+(2n+1)a^n
问题描述:
求和:1+3a+5a^2+…+(2n+1)a^n
答
设S=1+3a+5a^2+…+(2n+1)a^n
则aS=a+3a^2+5a^3+…+(2n+1)a^(n+1)
两式相减下就容易了~