Đáp án:
$\begin{array}{l}
a)\left( {\sqrt {18} + \sqrt {20} - \sqrt 8 } \right).\sqrt 2 - 2\sqrt {10} \\
= \left( {3\sqrt 2 + 2\sqrt 5 - 2\sqrt 2 } \right).\sqrt 2 - 2\sqrt {10} \\
= \left( {\sqrt 2 + 2\sqrt 5 } \right).\sqrt 2 - 2\sqrt {10} \\
= 2 + 2\sqrt {10} - 2\sqrt {10} \\
= 2\\
b)\sqrt {{{\left( {\sqrt 3 - 3} \right)}^2}} - \sqrt {12 + 6\sqrt 3 } \\
= 3 - \sqrt 3 - \sqrt {9 + 2.3.\sqrt 3 + 3} \\
= 3 - \sqrt 3 - \sqrt {{{\left( {3 + \sqrt 3 } \right)}^2}} \\
= 3 - \sqrt 3 - 3 - \sqrt 3 \\
= - 2\sqrt 3 \\
c)\dfrac{3}{{\sqrt 5 - \sqrt 2 }} + \dfrac{2}{{2 + \sqrt 2 }} + \dfrac{{\sqrt 5 - 5}}{{\sqrt 5 - 1}}\\
= \dfrac{{3\left( {\sqrt 5 + \sqrt 2 } \right)}}{{5 - 2}} + \dfrac{{2\left( {2 - \sqrt 2 } \right)}}{{4 - 2}} + \dfrac{{\sqrt 5 \left( {\sqrt 5 - 1} \right)}}{{\sqrt 5 - 1}}\\
= \sqrt 5 + \sqrt 2 + 2 - \sqrt 2 + \sqrt 5 \\
= 2 + 2\sqrt 5
\end{array}$
