设Sn是等比数列{an}的前n项和,S3,S9,S6成等差数列,则公比q=______.
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设Sn是等比数列{an}的前n项和,S3,S9,S6成等差数列,则公比q=______.
设等比数列{an}的首项为a1,公比为q,
由S3,S9,S6成等差数列,知q≠1,且2S9=S3+S6,
即2
a1(1−q9)
1−q=
a1(1−q3)
1−q+
a1(1−q6)
1−q
整理得:2q6+q3=0,解得q=-
3
1
2
.
故答案为−
3
1
2
.
由S3,S9,S6成等差数列,知q≠1,且2S9=S3+S6,
即2
a1(1−q9)
1−q=
a1(1−q3)
1−q+
a1(1−q6)
1−q
整理得:2q6+q3=0,解得q=-
3
1
2
.
故答案为−
3
1
2
.
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