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$\displaystyle  \begin{array}{{>{\displaystyle}l}}
c) \ C\ =\ \sqrt{3-\sqrt{5}} \ .\left(\sqrt{10} \ -\ \sqrt{2}\right) .\left( 3+\sqrt{5}\right)\\
C\ =\ 2\ .\sqrt{3+\sqrt{5}} \ .\left(\sqrt{10} \ -\sqrt{2}\right)\\
C\ =\ 2.\ \left(\sqrt{30+10\sqrt{5}} \ -\ \sqrt{6+2\sqrt{5}}\right)\\
C\ =\ 2.(\sqrt{\left( 5+\sqrt{5}\right)^{2}} \ -\ \sqrt{\left(\sqrt{5} +1\right)^{2}}\\
C\ =\ 2.\left( 5+\sqrt{5} -\sqrt{5} -1\right)\\
C\ =\ 8\\
d) \ D\ =\sqrt{21\ -6\sqrt{6}} \ +\ \sqrt{9+2\sqrt{18}} \ -2\sqrt{6+3\sqrt{3}}\\
D\ =\ \sqrt{18\ -2.\sqrt{3} \ .\ 3\sqrt{2} \ +3} \ +\ \sqrt{9+2\sqrt{6} \ .\ \sqrt{3}} \ -2\sqrt{6+3\sqrt{3}}\\
D\ =\ \sqrt{\left( 3\sqrt{2} -\sqrt{3}\right)^{2}} \ +\ \sqrt{\left(\sqrt{6} +\sqrt{3}\right)^{2}} \ -2\sqrt{6+3\sqrt{3}}\\
D\ =\ \ 3\sqrt{2} \ -\sqrt{3} \ +\sqrt{6} +\sqrt{3} \ -2\sqrt{6+3\sqrt{3}}\\
D\ =\ 3\sqrt{2} \ +\sqrt{6} \ -2\sqrt{6+3\sqrt{3}}
\end{array}$