In the arithmetic sequence {an}, Sn is the sum of the first n terms, A2 + A8 = 18-a5, then S9=______ ．

Because the sequence {an} is an arithmetic sequence, A5 is the median of A2 and A8, so A2 + A8 = 2a5, from A2 + A8 = 18-a5, so 2a5 = 18-a5, so A5 = 6. In the arithmetic sequence {an}, S9 = (a1 + A9) × 92 = 9A5. So, S9 = 9 × 6 = 54

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