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7-2=5 15-7=8 26-15=118-5=3 11-8=3A_(n+2) - 2*A_(n+1) + A_n = 3 设 B_n = A_(n+1) - A_n则有:B_1 = 7-2 =5 B_2 = 15-7 = 8 .==>B_n = 5+(n-1)*3 = 3*n +2 sum(B_n) = A_(n+1) - A_1 = 5*n + (n-1)*n*3 ==>A_(n+1)...
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