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a2-a1=k a3-a2=k+d a4-a3=k+2d……an-a(n-1)=k+(n-2)d相加an-a1=(n-1)k+[1+2+……+(n-2)]d=(n-1)k+(n-2)(n-1)d/2所以an=a1+(n-1)k+(n-2)(n-1)d/2
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