1/a-1/b=5
(b-a)/ab=5
ab=-(a-b)/5
（3a-4ab-3b）/（a-2ab-b）=[3(a-b)-4ab]/[(a-b)-2ab]
=[3(a-b)+4(a-b)/5]/[(a-b)+2(a-b)/5]=(3+4/5)/(1+2/5)=19/7