设等差数列{an}的前n项和为Sn,且S5=a5.若a4≠0,则a7a4=_.

问题描述:

设等差数列{an}的前n项和为Sn,且S5=a5.若a4≠0,则

a7
a4
=______.

由已知S5=a5
∴a1+a2+a3+a4=0
∴a1+a4=a2+a3=0,
a1=−

3d
2

a7
a4
3d
2
+6d
3d
2
+3d
=3

故答案为3