Tìm x

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8. \( 2 \cos\left(\frac{x}{2}\right) \sqrt{3} = 0 \)

9. \( \cos 5x - \cos\left(2x - \frac{\pi}{6}\right) = 0 \)

10. \( \cos\left(2x + \frac{\pi}{3}\right) = -1 \)