用简便方法计算：6分之1+12分之1+20分之1+

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6分之1+12分之1+20分之1+30分之1+42分之1
=1/(2×3)+1/(3×4)+1/(4×5)+1/(5×6)+1/(6×7)
=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)
=1/2-1/7
=5/14