19: \(=\dfrac{4-2\sqrt{3}}{2}+\dfrac{4\sqrt{3}+4}{2}=\dfrac{8+2\sqrt{3}}{2}=4+\sqrt{3}\)
20: \(=\dfrac{-5+2\sqrt{6}-5-2\sqrt{6}}{1}=-10\)
21: \(=\dfrac{3\sqrt{2}+2-3-\sqrt{2}-3\sqrt{2}-3+2+\sqrt{2}}{\left(3+\sqrt{2}\right)\left(\sqrt{2}+1\right)}\)
\(=\dfrac{-2}{\left(3+\sqrt{2}\right)\left(\sqrt{2}+1\right)}\)