tochenshen
幼苗
共回答了18个问题采纳率:88.9% 举报
1/4+1/28+1/70+……+1/868
=1/(1×4)+1/(4×7)+1/(7×10)+……+1/(28×31)
=(1/3)×(1-1/4)+(1/3)×(1/4-1/7)+(1/3)×(1/7-1/10)+……+(1/3)×(1/28-1/31)
=(1/3)×[(1-1/4)+(1/4-1/7)+(1/7-1/10)+……+(1/28-1/31)]
=(1/3)×(1-1/4+1/4-1/7+1/7-1/10+……+1/28-1/31)
=(1/3)×(1-1/31)
=1/3×30/31
=10/31
1年前
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