1/(2×3)+1/(3×4)+1/(4×5)+……+1/(19×20)
=1/2-1/3+1/3-1/4+1/4-1/5+……+1/19-1/20
=1/2-1/20
=10/20-1/20
=9/20
1比2乘3+1比3乘4+1比4乘5+...+1比19乘20
1比2乘3+1比3乘4+1比4乘5+...+1比19乘20
数学人气：571 ℃时间：2019-10-19 00:47:04
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