#### Answer

all real x, $x\ne\frac{\pi}{4}+n\pi$ (n is an integer)

#### Work Step by Step

The function is defined for all real x where the denominator $1-\tan x\ne0$.
When $1-\tan x=0$,
$\tan x=1, x=\frac{\pi}{4}+n\pi$ (n is an integer)
$\therefore$ the domain is all real x, exluding $x=\frac{\pi}{4}+n\pi$ (n is an integer)