已知an等于(2n-1)*(1/3)^n,求sn
问题描述：
已知an等于(2n-1)*(1/3)^n,求sn
答
a(n)=(2n-1)×(1/3)^n则S(n)=1×(1/3)+3×(1/3)^2+…+(2n-1)×(1/3)^n(1/3)S(n)=1×(1/3)^2+3×(1/3)^3+…+(2n-3)×(1/3)^n+(2n-1)×(1/3)^(n+1)上式－下式,得(2/3)S(n)=1/3+2×(1/3)^2+2×(1/3)^3+…+2×(1/3)^n-(2...