Question
Find the principal value of ${\cot ^{ - 1}}\left( { \frac{-1}{{\sqrt 3 }}} \right)$
$\cot \theta = \frac{{ - 1}}{{\sqrt 3 }}$
We know that $\theta \in (0,\pi )$
$\cot \theta = \cot \left( {\pi - \frac{\pi }{3}} \right)$
$\theta = \frac{{2\pi }}{3}$
Therefore, principal value of ${\cot ^{ - 1}}\left( {\frac{{ - 1}}{{\sqrt 3 }}} \right) = \frac{{2\pi }}{3}$
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