Question

Write the principal value of ${\cos }^{-1}\left(\cos 680°\right)$

Answer 1

$$\begin{gathered} {\cos }^{-1}\left(\cos 680°\right)={\cos }^{-1}\left[\cos \left(720°-680°\right)\right] \\ ={\cos }^{-1}\left(\cos 40°\right) \\ =40° \end{gathered}$$