If $$2 \cos \theta=x+\frac{1}{x}$$, then $$2 \cos 3 \theta=$$

If $$\frac{\cos (A+B)}{\cos (A-B)}=\frac{\sin (C+D)}{\sin (C-D)}$$, then $$\tan A \tan B \tan C=$$

If $$\cos x=\frac{24}{25}$$ and $$x$$ lięs in first quadrant, then $$\sin \frac{x}{2}+\cos \frac{x}{2}=$$

The value of $$\sin 18^{\circ}$$ is