Question

Find the value of ${\cos }^{-1}(-1/2)$.

Answer 1

Let $y={\cos }^{-1}(-1/2)={\cos }^{-1}\left(\cos \frac{2\pi }{3}\right)=\frac{2\pi }{3}$
$[\because \cos (2\pi /3)=-1/2$ and ${\cos }^{-1}\left(\cos \theta \right)=\theta$ for $0\le \theta \le \pi ]$.