$$\frac { \cos 6 \theta - \cos 4 \theta } { \sin 6 \theta + \sin 4 \theta } = - \tan \theta$$
$\theta \notin \pi n_{1}+\frac{\pi }{2},\frac{\pi n_{1}}{5}$
$\forall n_{1}\in \mathrm{Z}$
