PT$\Leftrightarrow 4(\sin 5x-\sin x)(\sin 5x+\sin x)+2(\cos 6x+\cos 4x)+1=0$
$\Rightarrow 4(\sin 5x-\sin x)(\sin 5x+\sin x)+2(\cos 6x+\sin 4x)+1=0$
$\Leftrightarrow 4\sin 6x\sin 4x+2\cos 6x+2\sin 4x+1=0$
$\Leftrightarrow (2\cos 6x+1)(2\sin 4x+1)=0$ dễ tự giải