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In the arithmetic sequence {an}, A1 = 25, S17 = S9, find the general term formula of {an} (important process)
S9 = S17, so A10 + a11 + A12 + A13 + A14 + A15 + a16 + A17 = 0 {an} is an arithmetic sequence, so A10 + A17 = a11 + a16 = A12 + A15 = A13 + A14, so A13 + A14 = 0, that is, a1 + 12D + A1 + 13D = 2A1 + 25d = 0, because A1 = 25, so d = - 2A1 / 25 = - 2 * 25 / 25 = - 2, so an = a1 + (n-1) d = 27-2n
RELATED INFORMATIONS
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