1 câu trả lời
$\cos4x=\cos\dfrac{3 \pi}{ 5}$ $\Rightarrow 4x=\pm\dfrac{3\pi}{5} +k2\pi$ $\Rightarrow \left[ \begin{array}{l} x=\dfrac{3\pi}{20} +k\dfrac{\pi}{2}\\ x=-\dfrac{3\pi}{20} +k\dfrac{\pi}{2} \end{array} \right .(k\in\mathbb Z)$
$\cos4x=\cos\dfrac{3 \pi}{ 5}$ $\Rightarrow 4x=\pm\dfrac{3\pi}{5} +k2\pi$ $\Rightarrow \left[ \begin{array}{l} x=\dfrac{3\pi}{20} +k\dfrac{\pi}{2}\\ x=-\dfrac{3\pi}{20} +k\dfrac{\pi}{2} \end{array} \right .(k\in\mathbb Z)$
