The principle value of $cos^{-1}\left(-\frac{1}{2}\right)$ is :

$\frac{2\pi }{3}$ $-\frac{2\pi }{3}$ $\frac{\pi }{6}$ $-\frac{\pi }{6}$

$\frac{2\pi }{3}$

$\cos^{-1}\left(-\frac{1}{2}\right)=π-\left(\cos^{-1}\left(-\frac{1}{2}\right)\right)$ $=π-\frac{π}{3}$ $=\frac{2π}{3}$