Đáp án:
$\begin{array}{l}
2)x\left( {\sqrt x - y} \right) + \sqrt {xy} \left( {1 + \sqrt {xy} } \right)\\
= x\sqrt x - xy + \sqrt {xy} + xy\\
= x\sqrt x + \sqrt {xy} \\
3)3a - \left( {3\sqrt a - 1} \right)\left( {\sqrt a + 2} \right)\\
= 3a - \left( {3a + 6\sqrt a - \sqrt a - 2} \right)\\
= 3a - 3a - 5\sqrt a + 2\\
= - 5\sqrt a + 2\\
4)4\sqrt a \left( {\sqrt b - \dfrac{1}{4}b\sqrt a } \right) - \left( {\sqrt {ab} - 1} \right)\left( {\sqrt {ab} + 5} \right)\\
= 4\sqrt {ab} - ab - \left( {ab + 5\sqrt {ab} - \sqrt {ab} - 5} \right)\\
= 4\sqrt {ab} - ab - ab - 4\sqrt {ab} + 5\\
= 5
\end{array}$
