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PT
$\Leftrightarrow 2\sin\frac{7x}{2}\cos\frac{3x}{2} + 2\sin\frac{x}{2}\cos\frac{5x}{2} +2\sin2x\cos7x =0$
$\Leftrightarrow \sin 5x + \sin 2x + \sin 3x - \sin 2x +\sin 9x - \sin 5x =0$
$\Leftrightarrow \sin 3x +\sin 9x =0$
$\Leftrightarrow \sin 3x =\sin (-9x )$.
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