Đáp án:
$\begin{array}{l}
a)2\sqrt {112} - \frac{7}{6}\sqrt {252} - 5\sqrt {63} + 3\sqrt {28} \\
= 2.4\sqrt 7 - \frac{7}{6}.6\sqrt 7 - 5.3\sqrt 7 + 3.2\sqrt 7 \\
= 8\sqrt 7 - 7\sqrt 7 - 15\sqrt 7 + 6\sqrt 7 \\
= - 8\sqrt 7 \\
b)\sqrt {{{\left( {2\sqrt 5 - 6} \right)}^2}} - \sqrt {24 - 8\sqrt 5 } \\
= \left| {2\sqrt 5 - 6} \right| - \sqrt {20 - 2.2\sqrt 5 .2 + 4} \\
= 6 - 2\sqrt 5 - \sqrt {{{\left( {2\sqrt 5 - 2} \right)}^2}} \\
= 6 - 2\sqrt 5 - \left( {2\sqrt 5 - 2} \right)\\
= 8 - 4\sqrt 5
\end{array}$
