\[\frac{a^{2} - 4a}{2a^{2} - 50} - \frac{6a - 20}{2a^{2} - 50} + \frac{5}{2a^{2} - 50} =\]
\[= \frac{a^{2} - 4a - 6a + 20 + 5}{2\left( a^{2} - 25 \right)} =\]
\[= \frac{a^{2} - 10a + 25}{2\left( a^{2} - 25 \right)} =\]
\[= \frac{(a - 5)^{2}}{2(a - 5)(a + 5)} = \frac{a - 5}{2a + 10}\]