`a) 2cos 3x = 4`
`-> cos 3x = 2`
`text{Mà}` `cos 3x in [-1; 1]`
`-> text{Phương trình vô nghiệm}`
`b) sin 3x + cos 3x = sqrt{2}sin x`
`-> 1/(\sqrt{2})sin 3x + 1/(\sqrt{2})cos 3x = sin x`
`-> sin (3x + π/4) = sin x`
`->` \(\left[ \begin{array}{l}3x + \dfrac{\pi}{4} = x + k2\pi\\3x + \dfrac{\pi}{4} = \pi - x + k2\pi\end{array} \right.\)
`->` \(\left[ \begin{array}{l}x = -\dfrac{\pi}{8} + k\pi\\x = \dfrac{3\pi}{16} + k\dfrac{\pi}{2}\end{array} \right.\) `(k in ZZ)`