sn=1/a+2/a^2+…+n/a^n(a≠0),求sn

sn=1/a+2/a^2+…+n/a^n(a≠0),求sn 求和Sn=(a-1)+(a^2-2)+(a^3-3)+…+(a^n-n)?谢谢. 求和Sn=1/a+2/a^2+3/a^3+…+n/a^n 求和Sn=(a-1)+(a^2-2)+(a^3-3)+…+(a^n-n)? 求和Sn=(a-1)+(a^2-3)+(a^3-5)+...+(a^n-(2n-1)) 求和Sn=1／a+2／a^2+3／a^3+...+n／a^n Help!Sn是数列（a n)的前n项和,a n=(2n)^2 /(2n-1)(2n+1),求Sn 求和Sn=(a-1)+(a^2-3)+(a^3-50+…+[a^n-(2n-1)]. 设数列An的前n项和为Sn,已知a(1)+2a(2)+3a(3)+…+na(n)=(n-1)Sn+2n(n为正整数）.求证数列Sn+2是等比数列 求和Sn=2+5a+8a^2+.+[3n-1]*a^[n-1] 求和Sn=1/a+2/a+···+n/a的n次方 证明数列是等比数列数列前n项和为Sn,a1=1,a(n+1)=（n+2)Sn/n,求证Sn/n是等比数列, Sn=(a-1)+(a^2-1)+(a^3-1)+...+(a^n-1)求和 Sn=(a-1)+(a^2-1)+(a^3-1)+...+(a^n-1) 1/a(n)^2=4n-3 a(n)前n项和为Sn,证明2Sn＞-1+根号(4n+1) 设数列{a（n）}的前n项和为Sn,a（1）=2,S（n+1）=Sn-3,求a(n) 已知数列 an前n项和为Sn,a1=1,Sn=2a(n+1),求Sn 若数列a n=1/[n(n-1)×2^n].求前n项和Sn Sn=2n^2-3n+1 ,a(n)=