& \text{ si } \frac{\alpha}{2}\in [-\frac{\pi}{2},\,\,\frac{\pi}{2}\,)+2k\pi \\ -\sqrt{\frac{1+\cos\alpha}{2}} & \text{ si } \frac{\alpha}{2}\in [\;\;\;\frac{\pi}{2},\,\frac{3\pi}{2})+2k\pi \end{cases}\;, \;\;para\;k\in \mathbb{Z}