Find values of the parameter for which the expression does not depend on
%V0
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 $x.$
Izvor: Međunarodna matematička olimpijada, shortlist 1967
Školjka 
for which the expression 
