Đáp án:
$\begin{array}{l}
a)\dfrac{{x\sqrt x - y\sqrt y }}{{\sqrt x - \sqrt y }}\\
= \dfrac{{\left( {\sqrt x - \sqrt y } \right)\left( {x + \sqrt {xy} + y} \right)}}{{\sqrt x - \sqrt y }}\\
= x + \sqrt {xy} + y\\
b)\\
5a\sqrt {64a{b^3}} - \sqrt 3 .\sqrt {12{a^3}{b^3}} + 2ab\sqrt {9ab} - 5b\sqrt {81{a^3}b} \\
= 5a.8b\sqrt {ab} - 2.3.ab\sqrt {ab} + 2ab.3\sqrt {ab} - 5b.9a.\sqrt {ab} \\
= 40ab\sqrt {ab} - 6ab\sqrt {ab} + 6ab\sqrt {ab} - 45ab\sqrt {ab} \\
= - 5ab\sqrt {ab} \\
c)\dfrac{{x - \sqrt {3x} + 3}}{{x\sqrt x + 3\sqrt 3 }}\\
= \dfrac{{x - \sqrt {3x} + 3}}{{\left( {\sqrt x + \sqrt 3 } \right)\left( {x - \sqrt {3x} + 3} \right)}}\\
= \dfrac{1}{{\sqrt x + \sqrt 3 }}\\
= \dfrac{{\sqrt x - \sqrt 3 }}{{x - 3}}
\end{array}$
