a²-b²/a²-2ab+b²+2a-ab/b-2÷a
=（a+b）（a-b）/(a-b)²+a(2-b)/(b-2）÷a
=（a+b）/（a-b）-1
=(a+b-a+b)/(a-b)
=2b/(a-b)
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