等比数列{an}的首项a1=-1,前n项和为Sn,已知S10S5=3132,则a2等于(  ) A.23 B.−12 C.2 D.12

问题描述:

等比数列{an}的首项a1=-1,前n项和为Sn,已知

S10
S5
31
32
,则a2等于(  )
A.
2
3

B.
1
2

C. 2
D.
1
2

S10
S5
31
32

∴q5=
S10S5
S5
=
S10
S5
−1
=
31
32
-1=-
1
32

∴q=
1
2

则a2=a1q=(-1)×(-
1
2
)=
1
2

故选D