điều kiện:$cosx \neq 0$$tan^4x+1=\frac{(2-sin^22x)sin3x}{cos^4x}$
$\Leftrightarrow \frac{sin^4x+cos^4x}{cos^4x}-\frac{(2-sin^22x)sin3x}{cos^4x}=0$
$\Leftrightarrow 1-2sin^2xcos^2x-(2-sin^22x)sin3x=0$
$\Leftrightarrow \frac{2-sin^22x}{2}-(2-sin^22x)sin3x=0$
$\Leftrightarrow (2-sin^22x)(\frac{1}{2}-sin3x)=0$