Đk : $\sin 4x\neq 0$Pt đã cho $\Leftrightarrow \frac{-\cos 2x}{\frac{1}{2}\sin 2x}+\frac{2\sin 2x}{cos 2x}+\cot^3 4x=3$$\Leftrightarrow \frac{2\sin^22x-2\cos^22x}{\sin 2x\cos 2x}+\cot^34x=3$$\Leftrightarrow \frac{-4\cos 4x}{\sin 4x}+\cot^34x=3$$\Leftrightarrow \cot^34x-4\cot 4x-3=0$$\Leftrightarrow (\cot 4x+1)(\cot^24x-\cot 4x-3)=0$
Đk : $\sin 4x\neq 0$Pt đã cho $\Leftrightarrow \frac{1-2\cos^2 x}{\sin x\cos x}+\frac{\cos 2x}{\sin x\cos x}+\cot^3 4x=3$$\Leftrightarrow \frac{1-2\cos^2 x+2\cos^2x-1}{\sin x\cos x}+\cot^34x=3$$\Leftrightarrow \cot^34x=3$$\Leftrightarrow \cot 4x=\sqrt{3}$$\Leftrightarrow 4x=\pi/6+k\pi$$\Leftrightarrow x=\pi/24+k\pi/4$
Đk : $\sin 4x\neq 0$Pt đã cho $\Leftrightarrow \frac{-\cos
2x}{\
frac{1}{2}\sin
2x}+\frac{2\sin
2x
}{cos
2x}+\cot^3 4x=3$$\Leftrightarrow \frac{2\s
in^2
2x
-2\cos^2
2x}{\sin
2x\cos
2x}+\cot^34x=3$$\Leftrightarrow \
frac{-4\cos 4x}{\sin 4x}+\cot^34x=3$$\Leftrightarrow \cot
^34x
-4\
cot 4x
-3=
0$$\Leftrightarrow
(\cot 4x
+1)(\
cot^24
x-\
cot 4
x-3)=0$