\[f(x) = \frac{x^{2} + 4}{x^{2} - 10x + 24}\]
\[x^{2} - 10x + 24 \neq 0\]
\[x_{1} + x_{2} = 10;\ \ x_{1} \cdot x_{2} = 24\]
\[x_{1} \neq 4;\ \ x_{2} \neq 6\ \]
\[\text{D\ }(f) = ( - \infty;4) \cup (4;6) \cup (6;\ + \infty).\]