1/a-1/b=3(b-a)/ab=3b-a=3ab2a+3ab-2b/a-ab-b=(2a+b-a-2b)[a-b-(b-a)/3]=(a-b)/[4(a-b)/3]=3/4 或者2a+3ab-2b/a-ab-b=[2(-3ab)+3ab]/(-3ab-ab)=3/4
