We know the arithmetic sequence {an} A1 = 25, S17 = S9; The known arithmetic sequence {an} A1 = 25, S17 = S9 \X05 (1) the general formula of {an}; \x05(2)S1、S2、… Which one is the largest? And find the maximum

We know the arithmetic sequence {an} A1 = 25, S17 = S9; The known arithmetic sequence {an} A1 = 25, S17 = S9 \X05 (1) the general formula of {an}; \x05(2)S1、S2、… Which one is the largest? And find the maximum

S17 = s917a1 + 17 × 16d / 2 = 9a1 + 9 × 8D / 2A1 = 25, the general formula of 4D = - 8D = - 2An = a1 + (n-1) d = 25-2 (n-1) = 27-2n sequence {an} is an = 27-2nsn = Na1 + n (n-1) d / 2 = - N & # 178; + 26n = - (N-13) & # 178; + 169 when n = 13, Sn has the maximum (SN) max = 169

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