\(\cos4x=1\\\Leftrightarrow 4x=k2\pi\\\Leftrightarrow x=k\dfrac{\pi}{2}.(k\in\mathbb{Z})\\\text{Do }x\in(0;2\pi)\\\Rightarrow 0<x<2\pi\\\Leftrightarrow 0<k\dfrac{\pi}{2}<2\pi\\\Leftrightarrow 0<k<4\\\text{mà }k\in\mathbb{Z}\Rightarrow k\in\{1;2;3\}\\\Rightarrow x=\dfrac{\pi}{2};x=\pi;x=\dfrac{3\pi}{2}.\\\text{Vậy phương trình có 3 nghiệm trên khoảng }(0;2\pi)\text{ là :}\\x=\dfrac{\pi}{2};x=\pi;x=\dfrac{3\pi}{2}.\)
