$$\frac { \cos ( \pi + \theta ) \cos ( - \theta ) } { \cos ( \pi - \theta ) \cos ( \frac { \pi } { 2 } + \theta ) } = - \cot \theta$$
$\theta \neq \frac{\pi n_{1}}{2}$
$\forall n_{1}\in \mathrm{Z}$
