(tg a/ 1+tg^2a) - (ctg a / 1+ctg^2a)
[m]\frac{tg\alpha }{1+tg^2\alpha }-\frac{ctg\alpha }{1+ctg^2\alpha }=\frac{tg\alpha }{1+tg^2\alpha }-\frac{\frac{1}{tg\alpha }}{1+\frac{1}{tg^2\alpha }}=\frac{tg\alpha }{1+tg^2\alpha }-\frac{tg\alpha }{tg^2\alpha +1}=0[/m]