Question

Evaluate ${\cos }^{-1}\left[\cos 11\pi /6\right]$

Answer 1

$\begin{array}{c}{\cos }^{-1}\left[\cos 11\pi /6\right]\\ \Rightarrow {\cos }^{-1}\left[\cos \left(2\pi -\frac{\pi }{6}\right)\right]\\ \Rightarrow {\cos }^{-1}\left[\cos \left(\pi /6\right)\right]\left[0<\pi /6\pi \right]\\ =\pi /6\end{array}$