ĐKXĐ:
a. \(sinx-1\ne0\Rightarrow sinx\ne1\Rightarrow x\ne\dfrac{\pi}{2}+k2\pi\)
b. \(cos3x+1\ne0\Rightarrow3x\ne-1\Rightarrow3x\ne\pi+k2\pi\)
\(\Rightarrow x\ne\dfrac{\pi}{3}+\dfrac{k2\pi}{3}\)
c. \(\left\{{}\begin{matrix}cos3x\ne0\\2cosx-1\ne0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}3x\ne\dfrac{\pi}{2}+k\pi\\cosx\ne\dfrac{1}{2}\\\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}x\ne\dfrac{\pi}{6}+\dfrac{k\pi}{3}\\x\ne\pm\dfrac{\pi}{3}+k2\pi\end{matrix}\right.\)
d. \(cos\left(4x-\dfrac{\pi}{3}\right)\ne0\Rightarrow4x-\dfrac{\pi}{3}\ne\dfrac{\pi}{2}+k\pi\)
\(\Rightarrow x\ne\dfrac{5\pi}{24}+\dfrac{k\pi}{4}\)
.Nhưng khoảng từ pi/2 đến -pi/4 thì vẫn bao gồm đáp án A mà sao mình lại chọn ạ
