$$\alpha = \sin 36^\circ $$ is a root of which of the following equation?

The value of $$\cos \left( {{{2\pi } \over 7}} \right) + \cos \left( {{{4\pi } \over 7}} \right) + \cos \left( {{{6\pi } \over 7}} \right)$$ is equal to :

$$16\sin (20^\circ )\sin (40^\circ )\sin (80^\circ )$$ is equal to :

The value of 2sin (12$$^\circ$$) $$-$$ sin (72$$^\circ$$) is :