10. If $\cos \theta=-\frac{12}{13}, \theta \in\left(\pi, \frac{3}{2} \pi\right)$, then $\cos \left(\theta+\frac{\pi}{4}\right)=$
10. $-\frac{7 \sqrt{2}}{26}$ yields $\sin \theta=-\frac{5}{13}$, then use the angle addition formula.