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=______.

设等比数列{an}首项为a1,公比为q,易知q≠1,
由题意得

S3
a1(1−q3)
1−q
=3
S6
a1(1−q6)
1−q
=9

两式子相除得1+q3=3,解得q3=2,
将q3=2代入S3=
a1(1−q3)
1−q
=3,
a1(1−2)
1−q
=3,即-
a1
1−q
=3,
a1
1−q
=-3,
∴S9=
a1(1−q9)
1−q
=
a1
1−q
•[1-(q33]=-3•(1-23)=-3•(-7)=21.
故答案为:21.