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說明：
2,4,8 // 16,32,64 // 128,256,512
原為等比數列，現在改成每 3 個一組，則每組的總和之間是否有關連?
2 + 4 + 8 = 14
16 + 32 + 64 = 112
128 + 256 + 512 = 896
112/14 = 8
896/112 = 8
=> 各組之間仍為等比關係
以本題來說
(a1 + a2 + a3), (a4 + a5 + a6), (a7 + a8 + a9) 之間成等比關係
已知 a1 + a2 + a3 = 21
a4 + a5 + a6 = S6 - S3 = 105 - 21 = 84
=> 公比 r = 84/21 = 4
=> a7 + a8 + a9 = 84 * 4 = 336
S9 = 21 + 84 + 336 = 441
