#### Answer

{ x | x $\lt$ 0 or x $\gt$ 5 }

#### Work Step by Step

Since $\frac{1}{\sqrt[4] x}$ is valid only if x $\gt$ 0, we must have:
x² - 5x = x (x - 5) $\gt$ 0, which is valid if:
- x and (x - 5) are both positive → x $\gt$ 5; or
- x and (x - 5) are both negative → x $\lt$ 0
{ x | x $\lt$ 0 or x $\gt$ 5 }