2分之1＋6分之1＋12分之1＋20分之1＋30分之1=1/1*2+1/2*3+1/3*4+1/4*5+1/5*6=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6=1-1/6=5/6
1/2+1/6+1/12+1/20+1/30
=1/（1*2）+1/（2*3）+1/（3*4）+1/（4*5)+1/(5*6)
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)
=1-1/6
=5/6
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