(1-a-b/a-2b)÷a^2-4b^2/a^2-4ab+4b^2

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(1-a-b/a-2b)÷a^2-4b^2/a^2-4ab+4b^2

原式=[(a-2b-a+b)/(a-2b)]÷[(a+2b)(a-2b)/(a-2b)²]
=[-b/(a-2b)]÷[(a+2b)/(a-2b)]
=[-b(a-2b)]/[(a-2b)(a+2b)]
=-b/(a+2b)