1÷【(n(n+1)(n+2)】
=(1/2)×{1/[n(n+1)]-1/[(n+1)(n+2)]}
求和,中间项相消,
得1/2*(1/2-1/890)
=217/870
2/1*2*3+2/2*3*4+2/3*4*5+…+2/28*29*30
=1/1*2-1/2*3+1/2*3-1/3*4+1/3*4-…+1/28*29-1/29*30
=1/2-1/29*30
=435/870-1/870
=343/870
=217/435
(最后的343/870等于217/435是约分。)
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