MCQ
${\cos ^{ - 1}}\left( {\cos \frac{{7\pi }}{6}} \right) = $
  • A
    $\frac{{7\pi }}{6}$
  • $\frac{{5\pi }}{6}$
  • C
    $\frac{\pi }{6}$
  • D
    None of these

Answer

Correct option: B.
$\frac{{5\pi }}{6}$
b
(b) ${\cos ^{ - 1}}\left( {\cos \frac{{7\pi }}{6}} \right) = {\cos ^{ - 1}}\left\{ {\cos \left( {\pi + \frac{\pi }{6}} \right)} \right\}$

$= {\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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