Đáp án:
$\begin{array}{l}
c)\dfrac{3}{{\sqrt 5 + \sqrt 7 - \sqrt 2 }}\\
= \dfrac{{3\left( {\sqrt 5 - \sqrt 2 - \sqrt 7 } \right)}}{{{{\left( {\sqrt 5 - \sqrt 2 } \right)}^2} - {{\left( {\sqrt 7 } \right)}^2}}}\\
= \dfrac{{3\sqrt 5 - 3\sqrt 2 - 3\sqrt 7 }}{{5 - 2\sqrt 5 .\sqrt 2 + 2 - 7}}\\
= \dfrac{{3\sqrt 5 - 3\sqrt 2 - 3\sqrt 7 }}{{ - 2\sqrt {10} }}\\
= \dfrac{{3\sqrt {70} + 6\sqrt 5 - 15\sqrt 2 }}{{20}}\\
d)\dfrac{1}{{2 + \sqrt 5 + 2\sqrt 2 + \sqrt {10} }}\\
= \dfrac{1}{{2 + \sqrt 5 + \sqrt 2 \left( {2 + \sqrt 5 } \right)}}\\
= \dfrac{1}{{\left( {2 + \sqrt 5 } \right)\left( {\sqrt 2 + 1} \right)}}\\
= \dfrac{{\left( {\sqrt 2 - 1} \right)\left( {\sqrt 5 - 2} \right)}}{{\left( {5 - {2^2}} \right)\left( {2 - {1^2}} \right)}}\\
= \left( {\sqrt 2 - 1} \right)\left( {\sqrt 5 - 2} \right)\\
= \sqrt {10} - 2\sqrt 2 - \sqrt 5 + 2
\end{array}$
