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Mathematics in senior two. Let the sum of the first n terms of the geometric series An Sn, S3+S6=2S9, then the common ratio Q is equal to?

When q= 1, S3+S6=9a 1, 2S9 = 18A 1. Because a 1≠0, S3+S6≠2S9 is set to Q ≠ 65438. A1(1-q9)1-q, simplified as 2q9-q6-q3=0,

Because q≠0, 2q6-q3- 1=0, that is, (2Q3+ 1) (Q3- 1) = 0. Q≠ 1, so q3=- 12.

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