làm giúp mình với mình cảm ơn sinx . cosx . cos2x = 1/4

TXĐ: $D=R$

$sinx.cosx.cos2x=\dfrac{1}{4}$

$↔ \dfrac{1}{2}sin2x.cos2x=\dfrac{1}{4}$

$↔ sin2x.cos2x=\dfrac{1}{2}$

$↔ \dfrac{1}{2}sin4x=\dfrac{1}{2}$

$↔ sin4x=1$

$↔ 4x=\dfrac{\pi}{2}+k2\pi$

$↔ x=\dfrac{\pi}{8}+\dfrac{k\pi}{2}$ $(k∈Z)$