ĐKXĐ:
a. \(cosx\ne0\Leftrightarrow x\ne\dfrac{\pi}{2}+k\pi\)
b. \(cos2x\ne0\Leftrightarrow x\ne\dfrac{\pi}{4}+\dfrac{k\pi}{2}\)
c. \(sin\left(x+\dfrac{\pi}{4}\right)\ne0\Leftrightarrow x+\dfrac{\pi}{4}\ne k\pi\Leftrightarrow x\ne-\dfrac{\pi}{4}+k\pi\)