Question
${\cos ^{ - 1}}\left( {\cos \frac{{7\pi }}{6}} \right) = $
$= {\cos ^{ - 1}}\left( { - \cos \frac{\pi }{6}} \right) = \pi - {\cos ^{ - 1}}\cos \frac{\pi }{6} $
$= \pi - \frac{\pi }{6} = \frac{{5\pi }}{6}$.
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