Đáp án:
$\begin{array}{l}
2)\sqrt {4 - \sqrt 7 } + \sqrt {4 + \sqrt 7 } \\
= \frac{1}{{\sqrt 2 }}.\left( {\sqrt 2 .\sqrt {4 - \sqrt 7 } + \sqrt 2 .\sqrt {4 + \sqrt 7 } } \right)\\
= \frac{1}{{\sqrt 2 }}.\left( {\sqrt {8 - 2\sqrt 7 } + \sqrt {8 + 2\sqrt 7 } } \right)\\
= \frac{1}{{\sqrt 2 }}.\left( {\sqrt {{{\left( {\sqrt 7 - 1} \right)}^2}} + \sqrt {{{\left( {\sqrt 7 + 1} \right)}^2}} } \right)\\
= \frac{1}{{\sqrt 2 }}.\left( {\sqrt 7 - 1 + \sqrt 7 + 1} \right)\\
= \frac{1}{{\sqrt 2 }}.2\sqrt 7 \\
= \sqrt {14} \\
2)\sqrt {11 - 4\sqrt 6 } - \sqrt {11 + 4\sqrt 6 } \\
= \sqrt {8 - 2.2\sqrt 2 .\sqrt 3 + 3} - \sqrt {8 + 2.2\sqrt 2 .\sqrt 3 + 3} \\
= \sqrt {{{\left( {2\sqrt 2 - \sqrt 3 } \right)}^2}} - \sqrt {{{\left( {2\sqrt 2 + \sqrt 3 } \right)}^2}} \\
= 2\sqrt 2 - \sqrt 3 - 2\sqrt 2 - \sqrt 3 \\
= - 2\sqrt 3
\end{array}$
