\(D=\left(\dfrac{a+b\sqrt{a}+b-\sqrt{ab}}{\sqrt{a}+\sqrt{b}}\right):\left(\dfrac{a}{\sqrt{b}\left(\sqrt{a}+\sqrt{b}\right)}-\dfrac{b}{\sqrt{a}\left(\sqrt{a}-\sqrt{b}\right)}-\dfrac{a+b}{\sqrt{ab}}\right)\)
\(=\left(\dfrac{a+b\sqrt{a}+b-\sqrt{ab}}{\sqrt{a}+\sqrt{b}}\right):\dfrac{a\sqrt{a}\left(\sqrt{a}-\sqrt{b}\right)-b\sqrt{b}\left(\sqrt{a}+\sqrt{b}\right)-\left(a^2-b^2\right)}{\sqrt{ab}\left(a-b\right)}\)
\(=\left(\dfrac{a+b\sqrt{a}+b-\sqrt{ab}}{\sqrt{a}+\sqrt{b}}\right):\dfrac{a^2-a\sqrt{ab}-b\sqrt{ab}-b^2-a^2+b^2}{\sqrt{ab}\left(a-b\right)}\)
\(=\left(\dfrac{a+b\sqrt{a}+b-\sqrt{ab}}{\sqrt{a}+\sqrt{b}}\right):\dfrac{-\sqrt{ab}\left(a+b\right)}{\sqrt{ab}\left(a-b\right)}\)
\(=\dfrac{a+b+b\sqrt{a}-\sqrt{ab}}{\sqrt{a}+\sqrt{b}}\cdot\dfrac{-\left(a-b\right)}{a+b}\)
\(=\dfrac{-\left(a+b+b\sqrt{a}-\sqrt{ab}\right)\left(\sqrt{a}-\sqrt{b}\right)}{a+b}\)
