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Câu 3: Giải các phương trình sau:

a) \( 3\cos x + 7 = 0. \)

b) \( 2\sin 2x - \sqrt{2} = 0. \)

c) \( \cos \left( 2x - \frac{\pi}{3} \right) - 1 = 0. \)

d) \( 2\cos(2x - \frac{\pi}{4}) - \sqrt{3} = 0. \)

e) \( 3\tan \left( \frac{3x+\frac{\pi}{2}}{3} \right) + \sqrt{3} = 0. \)

f) \( \cos 5x + \cos 2x = 0. \)

g) \( 3 - \sqrt{5}\tan(30^\circ - x) = 0. \)

h) \( \sin 3x - \sin 2x = 0. \)

i) \( \sin 3x + \sin x = 0. \)