李cc
幼苗
共回答了23个问题采纳率：82.6% 举报
1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72
= (1/2 + 1/6 + 1/12) + (1/20 + 1/30) + (1/42 + 1/56 + 1/72)
= (6+2+1)/12 + (3+2)/60 + (72+54+42)/(6*7*8*9)
= 9/12 + 5/60 + 168/(6*7*8*9)
= 3/4 + 1/12 + 1/18
= (27+3+2)/36
= 8/9
1年前
5
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