7/6+13/12+21/20+31/30+.421/420
问题描述：
7/6+13/12+21/20+31/30+.421/420
答
7/6+13/12+21/20+31/30+.421/420
=1+1/6+1+1/12+1+1/20+...+1+1/420
=19×1+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+...+(1/20-1/21)
=19+1/2-1/3+1/3-1/4+1/4-1/5+...+1/20-1/21
=19+1/2-1/21
=19+21/42-2/42
=19又42分之19；
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