\[\sqrt{3x - 9} + \frac{1}{\sqrt{40 - 5x}}\]
\[\left\{ \begin{matrix} 3x - 9 \geq 0\ \ \ \\ 40 - 5x > 0 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} 3x \geq 9\ \ \\ 5x < 40 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} x \geq 3 \\ x < 8 \\ \end{matrix} \right.\ \]
\[Ответ:\lbrack 3;8).\]