\textbf{110-} The general solution of the trigonometric equation $\cos 2x = \sin x \sin 3x$ is which of the following?
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(1) $\dfrac{k\pi}{2} - \dfrac{\pi}{6}$ \hfill (2) $\dfrac{k\pi}{3} + \dfrac{\pi}{6}$ \hfill (3) $k\pi + \dfrac{\pi}{2}$ \hfill (4) $\dfrac{k\pi}{3}$
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