(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A=1/2+1/3+1/4
原式=[1+A][A+1/5]-[1+A+1/5]*A=A+1/5+AA+A*1/5-A-AA-A*1/5
=1/5
原式=1/5
设A=1+1/2+1/3+1/4
B=1/2+1/3+1/4+1/5
则原式可转化为：
AB-(A+1/5)(B-1/5)
=AB-(AB-A/5+B/5-1/25)
=AB-AB+(A-B)/5+1/25
=[1-(1/5)]/5+1/25
=6/25
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