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.
答
设等差数列{an}的公差为d,
则
,
a3=a1+2d=7
a5+a7=2a1+10d=26
解得
,
a1=3 d=2
∴an=3+2(n-1)=2n+1
Sn=
=n2+2nn(3+2n+1) 2