\[\sqrt{7 + 2\sqrt{10}} = \sqrt{5} + \sqrt{2}\]
\[Возведем\ в\ квадрат:\]
\[\left( \sqrt{7 + 2\sqrt{10}} \right)^{2}\text{=}\left( \sqrt{5} + \sqrt{2} \right)^{2}\]
\[7 + 2\sqrt{10} = 5 + 2\sqrt{5 \cdot 2} + 2\]
\[7 + 2\sqrt{10} = 7 + 2\sqrt{10}\]
\[ч.т.д.\]