\[\frac{5 - 2\sqrt{6}}{5 + 2\sqrt{6}} = 49 - 20\sqrt{6}\]
\[\frac{\left( 5 - 2\sqrt{6} \right)\left( 5 - 2\sqrt{6} \right)}{\left( 5 + 2\sqrt{6} \right)\left( 5 - 2\sqrt{6} \right)} =\]
\[= \frac{\left( 5 - 2\sqrt{6} \right)^{2}}{25 - 4 \cdot 6} =\]
\[= \frac{25 - 20\sqrt{6} + 24}{25 - 24} =\]
\[= 49 - 20\sqrt{6} \Longrightarrow ч.т.д.\]