28. (ITA 3) Find values of the parameter $u$ for which the expression
$$
y=\frac{\tan (x-u)+\tan x+\tan (x+u)}{\tan (x-u) \tan x \tan (x+u)}
$$
does not depend on $x$.
28. The given expression can be transformed into
$$
y=\frac{4 \cos 2 u+2}{\cos 2 u-\cos 2 x}-3 .
$$

It does not depend on $x$ if and only if $\cos 2 u=-1 / 2$, i.e., $u= \pm \pi / 3+k \pi$ for some $k \in \mathbb{Z}$.