If $630^{\circ}<\theta<810^{\circ}$ and $\tan \theta=-\frac{7}{24}$, then $\cos \left(\frac{\theta}{4}\right)=$

For $\theta \in\left[-\frac{\pi}{2}, \frac{\pi}{2}\right]$ if $2 \cos \theta+\sin \theta=1$ and $7 \cos \theta+6 \sin \theta=k$, then the possible values of $k$ are

$$ \sum\limits_{k=0}^{12} \frac{1}{\sin \left((k+1) \frac{\pi}{6}+\frac{\pi}{4}\right) \sin \left(\frac{k \pi}{6}+\frac{\pi}{4}\right)}= $$

If $\cos \alpha=\sec h \beta$, then $\beta=$