I am to calculate $\cos(v+\frac{\pi}{6})$ when $\cos v = -\frac{2}{3}$ and $0 \lt v \lt \pi$.
I know I can change $\cos(v+\frac{\pi}{6})$ into
$\cos v \times \cos \frac{\pi}{6} - \sin v \times \sin \frac{\pi}{6}$
but I fail to see how this helps me, or for that matter any other step I could take. Am I completely off?