If $$\tan A - \tan B = x$$ and $$\cot B - \cot A = y$$, then what is the value of $$\cot (A - B)$$ ?

What is the minimum value of $${{{a^2}} \over {{{\cos }^2}x}} + {{{b^2}} \over {{{\sin }^2}x}}$$ where a > 0 and b > 0 ?

What is $$\sin (\alpha + \beta ) - 2sin\alpha cos\beta + sin(\alpha - \beta )$$ equal to?

If $$\cos ec\theta = {{29} \over {21}}$$, where 0 < $$\theta$$ < 90$$^\circ$$, then what is the value of 4 sec$$\theta$$ + 4 tan$$\theta$$ ?