19:
\(=\dfrac{1}{\sqrt{2}}\cdot\left(\sqrt{7}+\sqrt{5}-\sqrt{7}+\sqrt{5}\right)=\sqrt{5}\)
21:
\(=\dfrac{1}{\sqrt{2}}\left(\sqrt{15}+1-\sqrt{15}+1-\sqrt{3}+1\right)\)
\(=\dfrac{3}{\sqrt{2}}-\sqrt{\dfrac{15}{2}}\)
24: \(=\left(\sqrt{3}+1\right)\left(\sqrt{3}-1\right)\left(-2+\sqrt{3}\right)\)
=2(-2+căn 3)
=-4+2căn 3
23: \(=\left(4+\sqrt{15}\right)\left(\sqrt{5}-\sqrt{3}\right)^2\)
=(4+căn 15)*(8-2căn 15)
=32-30=2