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

∵等差数列{an}的首项a1=1,公差d=3,
∴an=3n-2,Sn=na1+

n(n−1)
2
d=n+
3n(n−1)
2
=
n(3n−1)
2

Sn
n(3n−1)
2