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
答
由已知S5=a5
∴a1+a2+a3+a4=0
∴a1+a4=a2+a3=0,
∴a1=−
3d 2
∴
=a7 a4
=3−
+6d3d 2 −
+3d3d 2
故答案为3