Đáp án:
$\begin{array}{l}
a)\sqrt {169} + \dfrac{2}{5}\sqrt {25} - \dfrac{1}{3}\sqrt 9 \\
= 13 + \dfrac{2}{5}.5 - \dfrac{1}{3}.3\\
= 13 + 2 - 1\\
= 14\\
b)\sqrt {125} - \dfrac{1}{2}\sqrt {20} - \sqrt {80} \\
= 5\sqrt 5 - \dfrac{1}{2}.2\sqrt 5 - 4\sqrt 5 \\
= 5\sqrt 5 - \sqrt 5 - 4\sqrt 5 \\
= 0\\
c)\sqrt {75} - \sqrt {\dfrac{1}{3}} + \dfrac{{\sqrt {66} }}{{\sqrt {22} }}\\
= 5\sqrt 3 - \dfrac{{\sqrt 3 }}{3} + \sqrt 3 \\
= 6\sqrt 3 - \dfrac{1}{3}\sqrt 3 \\
= \dfrac{{17\sqrt 3 }}{3}\\
d)\dfrac{2}{{\sqrt 7 - 5}} - \dfrac{2}{{\sqrt 7 + 5}}\\
= \dfrac{{2\left( {\sqrt 7 + 5} \right) - 2\left( {\sqrt 7 - 5} \right)}}{{\left( {\sqrt 7 - 5} \right)\left( {\sqrt 7 + 5} \right)}}\\
= \dfrac{{20}}{{7 - 25}}\\
= \dfrac{{20}}{{ - 18}}\\
= \dfrac{{ - 10}}{9}
\end{array}$
