2) ${\sin }^2x+\dfrac{1}{4}{\sin}^23x-\sin x{\sin}3x=0$
$\Rightarrow (\sin x-\dfrac{1}{2}\sin3x)^2=0$
$\Rightarrow\sin x-\dfrac{1}{2}\sin 3x=0$
$\Rightarrow \sin x-\dfrac{1}{2}(3\sin x-4{\sin}^3x)=0$
$\Rightarrow \sin x=0$
$\Rightarrow x=k\pi(k\in\mathbb Z)$
Hoặc $\sin x=\dfrac{1}{2}\Rightarrow x=\dfrac{\pi}{6}+k2\pi$ và $x=\dfrac{5\pi}{6}+k2\pi(k\in\mathbb Z)$
Hoặc $\sin x=\dfrac{-1}{2}\Rightarrow x=\dfrac{-\pi}{6}+k2\pi$ và $x=\dfrac{7\pi}{6}+k2\pi(k\in\mathbb Z)$
