等比数列{an}中,S3=3,S6=9,则S9=_.
问题描述:
等比数列{an}中,S3=3,S6=9,则S9=______.
答
设等比数列{an}首项为a1,公比为q,易知q≠1,
由题意得
,
S3=
=3
a1(1−q3) 1−q
S6=
=9
a1(1−q6) 1−q
两式子相除得1+q3=3,解得q3=2,
将q3=2代入S3=
=3,
a1(1−q3) 1−q
得
=3,即-
a1(1−2) 1−q
=3,a1 1−q
=-3,a1 1−q
∴S9=
=
a1(1−q9) 1−q
•[1-(q3)3]=-3•(1-23)=-3•(-7)=21.a1 1−q
故答案为:21.