Đáp án:
$\begin{array}{l}
8)2\sqrt {12} + 5\sqrt {108} - 3\sqrt {48} \\
= 2.2\sqrt 3 + 5.6\sqrt 3 - 3.4\sqrt 3 \\
= 4\sqrt 3 + 30\sqrt 3 - 12\sqrt 3 \\
= 22\sqrt 3 \\
9)\\
\left( {2\sqrt 6 + \sqrt 5 } \right).\left( {2\sqrt 6 - \sqrt 5 } \right)\\
= {\left( {2\sqrt 6 } \right)^2} - 5\\
= 24 - 5\\
= 19\\
10)\left( {\sqrt {20} - 3\sqrt {10} + \sqrt 5 } \right).\sqrt 5 + 15\sqrt 2 \\
= \sqrt {100} - 3.5\sqrt 2 + 5 + 15\sqrt 2 \\
= 10 - 15\sqrt 2 + 5 + 15\sqrt 2 \\
= 15\\
11)\\
\left( {2\sqrt 3 + \sqrt 5 } \right).\sqrt 3 .\sqrt {160} \\
= \left( {2\sqrt 3 + \sqrt 5 } \right).4.\sqrt {30} \\
= 2\sqrt 3 .4\sqrt {30} + 4\sqrt 5 .\sqrt {30} \\
= 24\sqrt {10} + 20\sqrt 6 \\
12)\\
\left( {\sqrt {28} - \sqrt {12} - \sqrt 7 } \right).\sqrt 7 + 2\sqrt {21} \\
= \left( {2\sqrt 7 - 2\sqrt 3 - \sqrt 7 } \right).\sqrt 7 + 2\sqrt {21} \\
= \left( {\sqrt 7 - 2\sqrt 3 } \right).\sqrt 7 + 2\sqrt {21} \\
= 7 - 2\sqrt {21} + 2\sqrt {21} \\
= 7
\end{array}$
