(2014•河西区三模)设Sn为等比数列{an}的前n项和,8a2+a5=0,则S5S2等于( ) A.11 B.5 C.-8 D.-11
问题描述:
(2014•河西区三模)设Sn为等比数列{an}的前n项和,8a2+a5=0,则
等于( )S5 S2
A. 11
B. 5
C. -8
D. -11
答
设等比数列{an}的公比为q,(q≠0)
由题意可得8a2+a5=8a1q+a1q4=0,解得q=-2,
故
=S5 S2
=
a1(1−q5) 1−q
a1(1−q2) 1−q
=1−q5
1−q2
=-111−(−2)5
1−(−2)2
故选D