9.126. $y=\sqrt{\log _{\frac{1}{2}}^{2}(x-3)-1}$.
## Solution.

We obtain the inequality $\log _{\frac{1}{2}}^{2}(x-3)-1 \geq 0, \log _{\frac{1}{2}}^{2}(x-3) \geq 1 \Leftrightarrow$

$$
\Leftrightarrow\left[\begin{array} { l } 
{ \log _ { \frac { 1 } { 2 } } ( x - 3 ) \geq 1 , } \\
{ \log _ { \frac { 1 } { 2 } } ( x - 3 ) \leq - 1 }
\end{array} \Leftrightarrow \left[\begin{array} { l } 
{ 0 < x - 3 \leq \frac { 1 } { 2 } , } \\
{ x - 3 \geq 2 }
\end{array} \Leftrightarrow \left[\begin{array}{l}
3<x \leq \frac{7}{2} \\
x \geq 5
\end{array}\right.\right.\right.
$$

Answer: $x \in\left(3 ; \frac{7}{2}\right] \cup[5 ; \infty)$.