Sửa đề: \(D=\left(\sqrt{14}-\sqrt{6}\right)\cdot\left(5+\sqrt{21}\right)\cdot\sqrt{4-\sqrt{7}}\)
Ta có: \(D=\left(\sqrt{14}-\sqrt{6}\right)\cdot\left(5+\sqrt{21}\right)\cdot\sqrt{4-\sqrt{7}}\)
\(=\sqrt{2}\left(\sqrt{7}-\sqrt{3}\right)\cdot\left(5+\sqrt{21}\right)\cdot\sqrt{4-\sqrt{7}}\)
\(=\sqrt{8-2\sqrt{7}}\cdot\left(\sqrt{7}-\sqrt{3}\right)\cdot\left(5+\sqrt{21}\right)\)
\(=\left(\sqrt{7}-1\right)\cdot\left(\sqrt{7}-\sqrt{3}\right)\cdot\left(5+\sqrt{21}\right)\)
\(=\left(7-\sqrt{21}-\sqrt{7}+\sqrt{3}\right)\cdot\left(5+\sqrt{21}\right)\)
\(=35+7\sqrt{21}-5\sqrt{21}-21-5\sqrt{7}-7\sqrt{3}+5\sqrt{3}+3\sqrt{7}\)
\(=14+2\sqrt{21}-2\sqrt{7}-2\sqrt{3}\)
