1/a-1/b=1
则b/(ab)-a/(ab)=1
则(b-a)/(ab)=1
则b-a=ab
则a-b=-ab
2a+ab-2b/a-3ab-b
=2(a-b)+ab/(a-b)-3ab
=-2ab+ab/-ab-3ab
=-ab/-4ab
=1/4