Giải phương trình $\displaystyle 8\cot 2x=\frac{\left( {{\cos }^{2}}x-{{\sin }^{2}}x \right).\sin 2x}{{{\cos }^{6}}x+{{\sin }^{6}}x}$.
A. $\displaystyle x=-\frac{\pi }{4}+k\pi $
B. $\displaystyle x=\pm \frac{\pi }{4}+\frac{k\pi }{2}$
C. $\displaystyle x=\frac{\pi }{4}+k\pi $
D. $\displaystyle x=\frac{\pi }{4}+\frac{k\pi }{2}$