已知等差数列{an},首项a1=1,公差d=3,求通项an及前n项的和Sn.
问题描述:
已知等差数列{an},首项a1=1,公差d=3,求通项an及前n项的和Sn.
答
∵等差数列{an}的首项a1=1,公差d=3,
∴an=3n-2,Sn=na1+
d=n+n(n−1) 2
=3n(n−1) 2
.n(3n−1) 2
即Sn=
.n(3n−1) 2