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a2-a1=4 a3-a2=5 . an-a(n-1)=n+2 以上等式相加得 an-a1=4+5+.+n+2 an-5=(4+n+2)*(n-1)/2 an=(n+6)*(n-1)/2+5 an=(n^2+5n+4)/2 an=(n+1)(n+4)/2
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