Sn是公差不等于0的等差数列{an}的前n项和,若S5=40,且a1,a3,a7成等比数列,则an= _ .

问题描述:

Sn是公差不等于0的等差数列{an}的前n项和,若S5=40,且a1,a3,a7成等比数列,则an= ___ .

设等差数列的公差为:d
则有条件得;

5a1+
5×(5-1)
2
•d=40
(a1+2d)2=a1(a1+6d)

a1=8
d=0
(舍),
a1=4
d=2

∴an=a1+(n-1)d=2n+2.
故答案为:2n+2.