$$\frac{1-\tan\ \theta}{1+\tan\ \theta}=\frac{\cot\ \theta-1}{\cot\ \theta+1}$$
$\theta \notin \pi n_{1}+\frac{3\pi }{4},\frac{\pi n_{1}}{2}$
$\forall n_{1}\in \mathrm{Z}$
