2分之1-6分之1-12分之1-20分之1-30分之1-42分之1
=(1-1/2)-(1/2-1/3)-(1/3-1/4)-...-(1/6-1/7)
=1-1/2+1/2-1/3+1/3-...+1/6-1/7
=1-1/7
=6/7
很高兴为你解答，不懂请追问！满意请采纳，谢谢！
=1/2-(1/6+1/12+1/20+1/30+1/42)
=1/2-(1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7)
=1/2-(1/2-1/7)
=1/7
=1/1*2-1/2*3-1/3*4-1/4*5-1/5*6-1/6*7
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=6/7
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