The value of $$\tan 9^\circ - \tan 27^\circ - \tan 63^\circ + \tan 81^\circ $$ is equal to

The value of $$\sqrt 3 \cos ec20^\circ - \sec 20^\circ $$ is equal to

Angle $$\alpha$$ is divided into two parts A and B such that A $$-$$ B = x and tan A : tan B = p : q. The value of sin x is equal to

$$\sqrt {1 + \sin A} = - \left( {\sin {A \over 2} + \cos {A \over 2}} \right)$$ is true, if