ĐK: $a,b\geq 0;a\neq b$
$A=\dfrac{(\dfrac{a-b}{\sqrt{a}+\sqrt{b}})^{3}+2a\sqrt{a}+b\sqrt{b}}{3a^{2}+3b\sqrt{ab}}+\dfrac{\sqrt{ab}-a}{a\sqrt{a}-b\sqrt{a}}
\\=\dfrac{(\sqrt{a}-\sqrt{b})^3+2a\sqrt{a}+b\sqrt{b}}{3\sqrt{a}(a\sqrt{a}+b\sqrt{b})}-\dfrac{\sqrt{a}(\sqrt{a}-\sqrt{b})}{\sqrt{a}(a-b)}
\\=\dfrac{a\sqrt{a}-3a\sqrt{b}+3b\sqrt{a}-b\sqrt{b}+2a\sqrt{a}+b\sqrt{b}}{3\sqrt{a}(\sqrt{a}+\sqrt{b})(a-\sqrt{ab}+b)}-\dfrac{\sqrt{a}(\sqrt{a}-\sqrt{b})}{\sqrt{a}(\sqrt{a}-\sqrt{b})(\sqrt{a}+\sqrt{b})}
\\=\dfrac{3a\sqrt{a}-3a\sqrt{b}+3b\sqrt{a}}{3\sqrt{a}(\sqrt{a}+\sqrt{b})(a-\sqrt{ab}+b)}-\dfrac{1}{\sqrt{a}+\sqrt{b}}
\\=\dfrac{3\sqrt{a}(a-\sqrt{ab}+b)}{3\sqrt{a}(\sqrt{a}+\sqrt{b})(a-\sqrt{ab}+b)}-\dfrac{1}{\sqrt{a}+\sqrt{b}}
\\=\dfrac{1}{\sqrt{a}+\sqrt{b}}-\dfrac{1}{\sqrt{a}+\sqrt{b}}=0$
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