设等差数列{an}的前n项和为Sn,若a5=5a3,则S9S5=_.
问题描述:
设等差数列{an}的前n项和为Sn,若a5=5a3,则
=______. S9 S5
答
∵{an}为等差数列,
S9=a1+a2+…+a9=9a5,S5=a1+a2+…+a5=5a3,
∴
=S9 S5
=99a5
5a3
故答案为9