Normal
5/Gọi \[A=\left(\sqrt{7}+\sqrt{3} \right).\sqrt{5-\sqrt{21}} \]\[ \Leftrightarrow {A}^{2}={\left(\sqrt{7}+\sqrt{3} \right)}^{2}.\left(5-\sqrt{21} \right)\]\[ \Leftrightarrow {A}^{2}=\left(10+2\sqrt{21} \right).\left(5-\sqrt{21} \right)\]\[ \Leftrightarrow {A}^{2}=2\left(5+\sqrt{21} \right)\left(5-\sqrt{21} \right)\]\[ \Leftrightarrow {A}^{2}=2\left({5}^{2}-21 \right)=2.4=8\]\[ \Rightarrow A=2\sqrt{2}\].
5/
Gọi \[A=\left(\sqrt{7}+\sqrt{3} \right).\sqrt{5-\sqrt{21}} \]
\[ \Leftrightarrow {A}^{2}={\left(\sqrt{7}+\sqrt{3} \right)}^{2}.\left(5-\sqrt{21} \right)\]
\[ \Leftrightarrow {A}^{2}=\left(10+2\sqrt{21} \right).\left(5-\sqrt{21} \right)\]
\[ \Leftrightarrow {A}^{2}=2\left(5+\sqrt{21} \right)\left(5-\sqrt{21} \right)\]
\[ \Leftrightarrow {A}^{2}=2\left({5}^{2}-21 \right)=2.4=8\]
\[ \Rightarrow A=2\sqrt{2}\].
