Question
The principal solution of $\cos ^{-1}\left(\cos \left(\frac{9 \pi}{4}\right)\right)$ is

Answer

$\text { (d): In }[0, \pi]$
$\cos ^{-1}\left(\cos \left(\frac{9 \pi}{4}\right)\right)=\cos ^{-1}\left(\cos \left(2 \pi+\frac{\pi}{4}\right)\right)$
$=\left(\cos ^{-1}\left(\cos \frac{\pi}{4}\right)\right)=\frac{\pi}{4}$

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