Đáp án:
$\begin{array}{l}
c)\sqrt {45} + 2\sqrt 5 - \sqrt {20} \\
= \sqrt {9.5} + 2\sqrt 5 - \sqrt {4.5} \\
= \sqrt {{3^2}} .\sqrt 5 + 2\sqrt 5 - \sqrt {{2^2}} .\sqrt 5 \\
= 3\sqrt 5 + 2\sqrt 5 - 2\sqrt 5 \\
= 3\sqrt 5 \\
d)\left( {\sqrt {12} - 2\sqrt 3 + 5\sqrt 2 - \dfrac{3}{4}\sqrt 8 } \right).2\sqrt 6 \\
= \left( {\sqrt {4.3} - 2\sqrt 3 + 5\sqrt 2 - \dfrac{3}{4}.\sqrt {4.2} } \right).2\sqrt 6 \\
= \left( {2\sqrt 3 - 2\sqrt 3 + 5\sqrt 2 - \dfrac{3}{4}.2\sqrt 2 } \right).2\sqrt 6 \\
= \left( {5\sqrt 2 - \dfrac{3}{2}\sqrt 2 } \right).2\sqrt 6 \\
= \dfrac{{7\sqrt 2 }}{2}.2\sqrt 6 \\
= 7.\sqrt 2 .\sqrt 2 .\sqrt 3 \\
= 14\sqrt 3
\end{array}$
