\(\begin{array}{l}
a)\,\,\,Sai\,\,de\,\,vi\,\,{\left( { - \sqrt 2 } \right)^3} < 0\\
b)\,\,\,\frac{{3 + \sqrt 3 }}{{1 + \sqrt 3 }} - 6\sqrt {\frac{1}{3}} = \frac{{\sqrt 3 \left( {1 + \sqrt 3 } \right)}}{{1 + \sqrt 3 }} - \frac{{2.\sqrt 3 .\sqrt 3 }}{{\sqrt 3 }} = \sqrt 3 - 2\sqrt 3 = - \sqrt 3 \\
c)\sqrt {7 + 4\sqrt 3 } - \frac{1}{{2 + \sqrt 3 }} = \sqrt {4 + 4\sqrt 3 + 3} - \frac{1}{{2 + \sqrt 3 }}\\
= \sqrt {{{\left( {2 + \sqrt 3 } \right)}^2}} - \frac{1}{{2 + \sqrt 3 }} = 2 + \sqrt 3 - \frac{1}{{2 + \sqrt 3 }}
\end{array}\)
