Answer
$ -\frac{\pi}{6}$
Work Step by Step
$sin^{-1}(cos\frac{2\pi}{3})=sin^{-1}(-cos\frac{\pi}{3})=sin^{-1}(-sin(\frac{\pi}{2}-\frac{\pi}{3}))=sin^{-1}(-sin\frac{\pi}{6})=-\frac{\pi}{6}$.
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