已知等差数列{an}的前n项和为Sn,且满足:a3=6,a2+a5=14.求an及Sn.

问题描述:

已知等差数列{an}的前n项和为Sn,且满足:a3=6,a2+a5=14.求an及Sn

由题意可得

a1+2d=6
a1+d+a1+4d=14
,解得 
a1=2
d=2

故有an=2+(n-1)2=2n,Sn=na1+
n(n−1)
2
d
=n2+n.