1/a-1/b=2
通分
(b-a)/ab=2
b-a=2ab
a-b=-2ab
(2a+ab-2b)/(a-3ab-b)
=[2(a-b)+ab]/[(a-b)-3ab]
=(-4ab+ab)/(-2ab-3ab)
=-3ab/(-5ab)
=3/5