a. TXĐ: $\cos x\ne 1 \Leftrightarrow x\ne k2\pi,k\in\mathbb{Z}$
b. TXĐ: $\sin(x+\dfrac{\pi}{6})\ne 0 \Leftrightarrow x\ne-\dfrac{\pi}{6}+k\pi,k\in\mathbb{Z}$
c. TXĐ: $\cos(x-\dfrac{\pi}{3})\ne 0 \Leftrightarrow x\ne\dfrac{5\pi}{6}+k\pi,k\in\mathbb{Z}$