Answer
$ \cos ^{-1}\left( \cos \left( \dfrac {5\pi }{6}\right) \right) =\dfrac {5\pi }{6}$
Work Step by Step
$\cos ^{-1}\left( \cos x\right) =x\left( 0\leq x\leq \pi \right) $
$0\leq \dfrac {5\pi }{6}\leq \pi $
$\Rightarrow \cos ^{-1}\left( \cos \left( \dfrac {5\pi }{6}\right) \right) =\dfrac {5\pi }{6}$
