\[\sqrt{5 + 2\sqrt{6}} = \sqrt{2} + \sqrt{3}\]
\[Возведем\ в\ квадрат:\]
\[\left( \sqrt{5 + 2\sqrt{6}} \right)^{2} = \left( \sqrt{2} + \sqrt{3} \right)^{2}\]
\[5 + 2\sqrt{6} = 2 + 2 \cdot \sqrt{2} \cdot \sqrt{3} + 3\]
\[5 + 2\sqrt{6} = 5 + 2\sqrt{6}\]
\[\ ч.т.д.\]