\[x^{2} = \left( \sqrt{6 + 2\sqrt{5}} - \sqrt{6 - 2\sqrt{5}} \right)^{2}\]
\[6 + 2\sqrt{5} - 2\sqrt{\left( 6 + 2\sqrt{5} \right)\left( 6 - 2\sqrt{5} \right)} + 6 - 2\sqrt{5} =\]
\[= 12 - 2\sqrt{36 - 20} =\]
\[= 12 - 2 \cdot 4 = 12 - 8 = 4;\]
\[x^{2} = 4\]
\[x = \pm 2.\]
\[Что\ и\ требовалось\ доказать.\]