a/ ĐKXĐ: \(cos\left(x-\frac{\pi}{6}\right)\ne0\Rightarrow x-\frac{\pi}{6}\ne\frac{\pi}{2}+k\pi\Rightarrow x\ne\frac{2\pi}{3}+k\pi\)
b/ \(sin\left(3x-\frac{2\pi}{3}\right)\ne0\Rightarrow3x-\frac{2\pi}{3}\ne k2\pi\Rightarrow x\ne\frac{2\pi}{9}+\frac{k2\pi}{3}\)