1/a-1/b=2(b-a)/ab=2b-a=2aba-b=-2ab（3a+4ab-3b）÷（2a-3ab-2b）=[3(a-b)＋4ab]÷[2（a-b）-3ab]=(-6ab＋4ab)÷（-4ab-3ab）=2ab/7ab=2/7
