求数列3x,5x^2,7x^3,...,(2n+1)x^n,...的前n项和
问题描述：
求数列3x,5x^2,7x^3,...,(2n+1)x^n,...的前n项和
答
Sn=3x+5x^2+7x^3+...+(2n+1)x^n
xSn=3x^2+5x^3+7x^4+...+(2n+1)x^(n+1)
(1-x)Sn=3x+2(x^2+x^3+...+x^n)-(2n+1)x^(n+1)
剩下的应该会了吧