已知等差数列{an}满足:a3=7,a5+a7=26,{an}的前n项和为Sn.求an及Sn.

问题描述:

已知等差数列{an}满足:a3=7,a5+a7=26,{an}的前n项和为Sn.求an及Sn

设等差数列{an}的公差为d,

a3=a1+2d=7
a5+a7=2a1+10d=26

解得
a1=3
d=2

∴an=3+2(n-1)=2n+1
Sn=
n(3+2n+1)
2
=n2+2n