9.123. $y=\frac{\sqrt{4 x-x^{2}}}{\log _{3}|x-4|}$.
## Solution.

We obtain

$$
\left\{\begin{array} { l } 
{ 4 x - x ^ { 2 } \geq 0 , } \\
{ \operatorname { log } _ { 3 } | x - 4 | \neq 0 }
\end{array} \Leftrightarrow \left\{\begin{array} { l } 
{ x ( x - 4 ) \leq 0 , } \\
{ | x - 4 | \neq 1 , } \\
{ x - 4 \neq 0 }
\end{array} \Leftrightarrow \left\{\begin{array}{l}
0 \leq x \leq 4 \\
x \neq 3 \\
x \neq 5 \\
x \neq 4
\end{array}\right.\right.\right.
$$

Answer: $x \in[0 ; 3) \cup(3 ; 4)$.