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假定公差为 d 则有:a6 = a1 + 5da1 + a6 = 2a1 + 5d = 12 .........(1)a4 = a1 + 3d = 7 ...................(2)(2)*2 - (1)d = 2代入(2)得到 a1 = 1a9 = a1 + 8d = 1 + 8*2 = 17