计算
(2a/b)^2×1/（a-b）-a/b÷b/4
=4a^2/b^2×1/（a-b）-4a/b^2
=4a^2/[b^2(a-b)]-4a/b^2
=4a/b^2*[a/(a-b)-1]
=4a/b^2*b/(a-b)
=4a/[b(a-b)]
=4a/(ab-b^2)