设等差数列{an}的前n项和为Sn.若a5=5a3,则S9S5=( ) A.10 B.9 C.12 D.5
问题描述:
设等差数列{an}的前n项和为Sn.若a5=5a3,则
=( )S9 S5
A. 10
B. 9
C. 12
D. 5
答
∵等差数列{an},a5=5a3,
∴a1=-
d,3 2
∴
=9,S9 S5
故选:B.