\[\sqrt{7 + 4\sqrt{3}} = \sqrt{3} + 2\]
\[Возведем\ в\ квадрат:\]
\[\left( \sqrt{7 + 4\sqrt{3}} \right)^{2} = \left( \sqrt{3} + 2 \right)^{2}\]
\[7 + 4\sqrt{3} = 3 + 4\sqrt{3} + 4\]
\[7 + 4\sqrt{3} = 7 + 4\sqrt{3}\]
\[\ ч.т.д.\]