an=n(n+1) 求sn

已知an=1/2n(n+1),求Sn An=n×2^(n-1),求Sn an=（2^n-1）n,求Sn 已知an=(2n+1)*3^n,求Sn Sn+an=（n-1）／n（n+1）,求an 在数列{an}中,an=1/n(n+1)(n+2),求Sn的极限 已知an=5n(n+1)(n+2)(n+3),求数列{an}的前n项和Sn 等比数列an的前n项和为sn,sn=1+3an,求：an Sn是数列an的前n项和，an=1/n(n+2),求Sn 数列{an}前n项和为Sn,对一切正整数n都有Sn=n+(1/2)an,求an,Sn 数列{an}前n项和为Sn,且2Sn+1=3an,求an及Sn 高中数列求和An=1/n,求Sn.