Question

Write the principal value of \[\cos^{- 1} \left( \cos680^\circ  \right)\]

Answer 1

\[\cos^{- 1} \left( \cos680^\circ\right) = \cos^{- 1} \left[ \cos\left( 720^\circ - 680^\circ \right) \right]\]
\[ = \cos^{- 1} \left( \cos40^\circ \right)\]
\[ = 40^\circ\]