Question
Find the value of the expression ${\sin ^{ - 1}}\;\sin \left( {\frac{{2\pi }}{3}} \right)$.

Answer

${\sin ^{ - 1}}\left( {\sin \frac{{2\pi }}{3}} \right) \ne \frac{{2\pi }}{3}$
as $\frac{{2\pi }}{3} \in \left[ {\frac{{ - \pi }}{2},\frac{\pi }{2}} \right]$
${\sin ^{ - 1}}\left( {\sin \frac{{2\pi }}{3}} \right) = {\sin ^{ - 1}}\left[ {{{\sin }^{ - 1}}\left( {\pi - \frac{\pi }{3}} \right)} \right]$
$= {\sin ^{ - 1}}\left[ {\sin \left( {\frac{\pi }{3}} \right)} \right]$
$ = \frac{\pi }{3}$

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