知Sn=n²+2n+1求an

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 已知数列an的前n项和Sn,求数列的通项公式.(1)Sn=3n²-n (2)Sn=2n+1 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 已知数列{an},满足a1=1/2,Sn=n²×an,求an Sn是数列an的前n项和，an=1/n(n+2),求Sn 已知等差数列{an}的前n项和为Sn,且(2n-1)Sn+1 -(2n+1)Sn=4n²-1(n∈N*) 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