Question

Write the principal value of ${\cos }^{-1}\left[\cos \right({680}^o\left)\right]$.

Answer 1

${\cos }^{-1}\left[\cos \right({680}^o\left)\right]$

$={\cos }^{-1}\left[\cos \right(2\times 360-40\left)\right]$

$={\cos }^{-1}\left[\cos {40}^o\right]$

$={40}^o$