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PT $\Leftrightarrow \cos 3x=\sin 5x \Leftrightarrow \cos 3x= \cos \left ( \frac{\pi}{2} -5x\right )$
$\Leftrightarrow \left[ {\begin{matrix} 3x= \frac{\pi}{2} -5x+k2\pi\\ 3x=- \frac{\pi}{2} +5x+k2\pi \end{matrix}} \right.$
$\Leftrightarrow \left[ {\begin{matrix} x= \frac{\pi}{16}+k\frac{\pi}{4}\\ x= \frac{\pi}{4} -k\pi \end{matrix}} \right. (k \in \mathbb{Z}).$
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