cos4x = 1 thì x bằng mấy ,đơn vị pi

`~rai~`

\(\cos4x=1\\\Leftrightarrow 4x=k2\pi\\\Leftrightarrow x=k\dfrac{\pi}{2}.(k\in\mathbb{Z})\\\text{Vậy S=}\left\{k\dfrac{\pi}{2}\Big|k\in\mathbb{Z}\right\}.\)