a) $A=\sqrt{(2-\sqrt{3}{)}^2}+2\sqrt{3}$

A= $|2-\sqrt{3}|+2\sqrt{3}$
$A=2-\sqrt{3}+2\sqrt{3}$
$A=2+\sqrt{3}$
b) $B=\sqrt{18}-2\sqrt{50}+3\sqrt{8}+\sqrt[3]{27}$
$B=\sqrt{9.2}-2\sqrt{25.2}+3\sqrt{4.2}+\sqrt[3]{\mathrm{3.3.3}}$
$B=3\sqrt{2}-2.5\sqrt{2}+3.2\sqrt{2}+3$
$B=3\sqrt{2}-10\sqrt{2}+6\sqrt{2}+3$
$B=3-\sqrt{2}$
c) $C=\genfrac{}{}{0ex}{}{4}{\sqrt{5}-1}-\genfrac{}{}{0ex}{}{10}{\sqrt{5}}+\genfrac{}{}{0ex}{}{\sqrt{125}}{\sqrt{5}}+\sqrt{2}\cdot \sqrt{\genfrac{}{}{0ex}{}{5}{2}}$
$C=\genfrac{}{}{0ex}{}{4\cdot (\sqrt{5}+1)}{(\sqrt{5}-1)(\sqrt{5}+1)}-\genfrac{}{}{0ex}{}{2\cdot 5}{\sqrt{5}}+\sqrt{\genfrac{}{}{0ex}{}{125}{5}}+\sqrt{2}\cdot \genfrac{}{}{0ex}{}{\sqrt{5}}{\sqrt{2}}$
$C=\genfrac{}{}{0ex}{}{4\cdot (\sqrt{5}+1)}{(\sqrt{5}{)}^2-1^2}-2\sqrt{5}+\sqrt{25}+\sqrt{5}$
$C=\genfrac{}{}{0ex}{}{4\cdot (\sqrt{5}+1)}{5-1}-2\sqrt{5}+5+\sqrt{5}$
$C=\genfrac{}{}{0ex}{}{4\cdot (\sqrt{5}+1)}{4}-\sqrt{5}+5$
$C=\sqrt{5}+1-\sqrt{5}+5$
$C=6$.