设等比数列an的前n项和为Sn,如果S3+S6=2S9,求数列公比q?
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设等比数列an的前n项和为Sn,如果S3+S6=2S9,求数列公比q?
答案是-1/2*3次根号下4
答案是-1/2*3次根号下4
![设等比数列an的前n项和为Sn,如果S3+S6=2S9,求数列公比q?](/uploads/image/z/16081578-18-8.jpg?t=%E8%AE%BE%E7%AD%89%E6%AF%94%E6%95%B0%E5%88%97an%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%2C%E5%A6%82%E6%9E%9CS3%2BS6%3D2S9%2C%E6%B1%82%E6%95%B0%E5%88%97%E5%85%AC%E6%AF%94q%3F)
S9=S3+A4+A5+A6+A7+A8+A9
S9=S6+A7+A8+a9
2S9=S3+S6
A4+A5+A6+A7+A8+A9+A7+A8+a9=0
(A4+A5+A6)+(A7+A8+a9)=0
A7=A4×q^3
A8=A5×q^3
A9=A6×q^3
(A4+A5+A6)+2(A4+A5+A6)×q^3=0
A4+A5+A6=A4(1+q+q^2)
等比数列中任意一项不等于0
A4≠0
1+q+q^2=(q+0.5)^2+0.75>0
A4+A5+A6≠0
1+2q^3=0
q=(-1/2)^(1/3)
S9=S6+A7+A8+a9
2S9=S3+S6
A4+A5+A6+A7+A8+A9+A7+A8+a9=0
(A4+A5+A6)+(A7+A8+a9)=0
A7=A4×q^3
A8=A5×q^3
A9=A6×q^3
(A4+A5+A6)+2(A4+A5+A6)×q^3=0
A4+A5+A6=A4(1+q+q^2)
等比数列中任意一项不等于0
A4≠0
1+q+q^2=(q+0.5)^2+0.75>0
A4+A5+A6≠0
1+2q^3=0
q=(-1/2)^(1/3)
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