1/3*4=1/12=1/3-1/4
1/4*5=1/20=1/4-1/5
1/5*6=1/30=1/5-1/6
1/6*7=1/42=1/6-1/7
原式=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1/3-1/7=4/21
1/(3*4)+1/(4*5)+1/(5*6)+1/(6*7)
=(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)
=1/3-1/7
=4/21
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1/3-1/7
=4/21
原式=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1/3-1/7
=4/21
1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7=4/21
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