a/1-b/1=1
ab/b-ab/a=ab/ab
b-a=ab
将 ab=b-a 带入 a-2ab-b/2a+3ab-2b
a-2(b-a)-b/2a+3(b-a)-2b=a-2b+2a-b/2a+3b-3a-2b=3a-3b/-a+b=3（a-b）/-（a-b）=-3/1