设等比数列{an}的前n项和为Sn,若S6S3=3,则S9S6=( ) A.12 B.73 C.83 D.1
问题描述:
设等比数列{an}的前n项和为Sn,若
=3,则S6 S3
=( )S9 S6
A.
1 2
B.
7 3
C.
8 3
D. 1
答
∵
=3,S6 S3
∴
=3 整理得,1+q3=2,
a1(1−q6) 1−q
a1(1−q3) 1−q
∴q3=2
∴
= S9 S6
=
a1(1−q9) 1−q
a1(1−q6) 1−q
7 3
故选B.