Đáp án:
$\begin{array}{l}
a)Dkxd:a > 0;b > 0;a \ne b\\
b)A = \dfrac{{{{\left( {\sqrt a + \sqrt b } \right)}^2} - 4\sqrt {ab} }}{{\sqrt a - \sqrt b }} - \dfrac{{a\sqrt b + b\sqrt a }}{{\sqrt {ab} }}\\
= \dfrac{{a + 2\sqrt {ab} + b - 4\sqrt {ab} }}{{\sqrt a - \sqrt b }} - \dfrac{{\sqrt {ab} \left( {\sqrt a + \sqrt b } \right)}}{{\sqrt {ab} }}\\
= \dfrac{{{{\left( {\sqrt a - \sqrt b } \right)}^2}}}{{\sqrt a - \sqrt b }} - \left( {\sqrt a + \sqrt b } \right)\\
= \sqrt a - \sqrt b - \sqrt a - \sqrt b \\
= - 2\sqrt b
\end{array}$
