PT 1 R\Leftrightarrow (1-\cos 4x)\cos 6x +1-\cos 6x =\sin 2x \sin 8x$$\Leftrightarrow 1-\cos 4x \cos 6x -\sin 2x \sin 8x =0$$\Leftrightarrow 2-\cos 10x -\cos 2x +\cos 10x -\cos 6x=0$$\Leftrightarrow \cos 6x +\cos 2x -2=0$$\Leftrightarrow 4\cos^3 2x -3\cos 2x +\cos 2x -2=0$$\Leftrightarrow 4\cos^3 2x -2\cos 2x -2=0$$\Leftrightarrow \cos 2x = 1 \Rightarrow x=k\pi;\ k\in Z$
PT 1
$\Leftrightarrow (1-\cos 4x)\cos 6x +1-\cos 6x =\sin 2x \sin 8x$$\Leftrightarrow 1-\cos 4x \cos 6x -\sin 2x \sin 8x =0$$\Leftrightarrow 2-\cos 10x -\cos 2x +\cos 10x -\cos 6x=0$$\Leftrightarrow \cos 6x +\cos 2x -2=0$$\Leftrightarrow 4\cos^3 2x -3\cos 2x +\cos 2x -2=0$$\Leftrightarrow 4\cos^3 2x -2\cos 2x -2=0$$\Leftrightarrow \cos 2x = 1 \Rightarrow x=k\pi;\ k\in Z$