[tex]\displaystyle\bf\\\frac{Cos6\alpha -Cos4\alpha }{Cos6\alpha +Cos4\alpha } =\frac{-2Sin\dfrac{\alpha +\beta }{2} Sin\dfrac{\alpha -\beta }{2} }{2Cos\dfrac{\alpha +\beta }{2} Cos\dfrac{\alpha -\beta }{2} }=-tg\frac{\alpha +\beta }{2} tg\frac{\alpha -\beta }{2}[/tex]