2a-b/a+b-b/a-b)除以a-2b/a+b
=[(2a-b)(a-b)-b(a+b)]/(a+b)(a-b)x(a+b)/(a-2b)
=(2a²-3ab+b²-ab-b²)/（a-b）(a-2b)
=2a(a-2b)/(a-b)(a-2b)
=2a/(a-b)