cos4x(cos4x+1)=0
<=> \(\left[{}\begin{matrix}cos4x=0\\cos4x+1=0\end{matrix}\right.\)
<=>\(\left[{}\begin{matrix}4x=\dfrac{\pi}{2}+k\pi\\4x=\pi+k2\pi\end{matrix}\right.\)
<=>\(\left[{}\begin{matrix}x=\dfrac{\pi}{8}+k\dfrac{\pi}{4}\\x=\dfrac{\pi}{4}+k\dfrac{\pi}{2}\end{matrix}\right.\)
