Giải phương trình a) sin5x + sin8x + sin3x = 0. b) 4cos^3 x + 3 căn bậc hai 2

a) Ta có

Vậy \(x = \frac{{k\pi }}{4}\), \(x = \frac{\pi }{5} + \frac{{k2\pi }}{5}\), \[x = \frac{\pi }{3} + \frac{{k2\pi }}{3}\left( {k \in \mathbb{Z}} \right)\].

b) Ta có:

\[4co{s^3}x + \;3\sqrt 2 sin2x = 8cosx\]

Vậy \(x = \frac{\pi }{2} + k\pi \), \(x = \frac{\pi }{4} + k2\pi \), \(x = \frac{{3\pi }}{4} + k2\pi \left( {k \in \mathbb{Z}} \right)\).