(1+1/2+1/3+1/4)(1/2+1/3+1/4+1/5)-(1+1/2+1/3+1/4+1/5)(1/2+1/3+1/4)
设1+1/2+1/3+1/4为y 1/2+1/3+1/4为x
=y(x+1/5)-(y+1/5)x
=yx+1/5y-yx-1/5x
=1/5Y-1/5x
=1/5(y-x)
=1/5*1
=1/5
(1+/2+1/3+1/4)×(1/3+1/4+1/5)-(1+/2+1/3+1/4+1/5)×(1/2+1/3+1/4
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