`~rai~`
\(\sqrt{6+2\sqrt{2-\sqrt{13+4\sqrt{3}}}}\\=\sqrt{6+2\sqrt{2-\sqrt{12+2.2\sqrt{3}+1}}}\\=\sqrt{6+2\sqrt{2-\sqrt{(2\sqrt{3}+1)^2}}}\\=\sqrt{6+2\sqrt{2-|2\sqrt{3}+1|}}\\=\sqrt{6+2\sqrt{2-2\sqrt{3}-1}}\\=\sqrt{6+2\sqrt{1-2\sqrt{3}}}\\\text{Vì 1<}2\sqrt{3}\Rightarrow 1-2\sqrt{3}<0\\\Rightarrow \text{Biểu thức vô nghĩa.}\)