Đáp án:
$\begin{array}{l}
4)a) - 3\sqrt 5 + \sqrt {45} - \sqrt {80} \\
= - 3\sqrt 5 + \sqrt {9.5} - \sqrt {16.5} \\
= - 3\sqrt 5 + 3\sqrt 5 - 4\sqrt 5 \\
= - 4\sqrt 5 \\
b)3\sqrt 2 - 4\sqrt {18} + 2\sqrt {32} - \sqrt {50} \\
= 3\sqrt 2 - 4.3\sqrt 2 + 2.4\sqrt 2 - 5\sqrt 2 \\
= 3\sqrt 2 - 12\sqrt 2 + 8\sqrt 2 - 5\sqrt 2 \\
= - 6\sqrt 2 \\
5)a)A = \dfrac{{3 + \sqrt 3 }}{{\sqrt 3 }} - \dfrac{2}{{\sqrt 3 - 1}}\\
= \dfrac{{\sqrt 3 \left( {\sqrt 3 + 1} \right)}}{{\sqrt 3 }} - \dfrac{{2\left( {\sqrt 3 + 1} \right)}}{{3 - 1}}\\
= \sqrt 3 + 1 - \left( {\sqrt 3 + 1} \right)\\
= 0\\
b)B = \dfrac{{x + \sqrt {xy} }}{{y + \sqrt {xy} }} = \dfrac{{\sqrt x \left( {\sqrt x + \sqrt y } \right)}}{{\sqrt y \left( {\sqrt y + \sqrt x } \right)}} = \dfrac{{\sqrt x }}{{\sqrt y }} = \dfrac{{\sqrt {xy} }}{y}
\end{array}$
