赞 sg19852003 幼苗
共回答了21个问题采纳率：90.5% 举报
12分之2加20分之2加30分之2加42分之2加56分之2加72分之2加90分之2
=2×(1/12+1/20+1/30+1/42+1/56+1/72+1/90)
=2×(1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10)
=2×(1/3-1/10)
=7/15
=2×(1/12+1/20+1/30+1/42+1/56+1/72+1/90)
=2×(1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10)
=2×(1/3-1/10)
=7/15
1年前
8
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