[tex]\displaystyle\bf\\1)\\\\Sin11x+Sin5x=2Sin\frac{11x+5x}{2} Cos\frac{11x-5x}{2} =2Sin8x Cos3x\\\\2)\\\\Cos14x-Cos8x=-2Sin\frac{14x-8x}{2} Sin\frac{14x+8x}{2} =-2Sin3x Sin11x\\\\3)\\\\Sin13x-Sin7x=2Sin\frac{13x-7x}{2} Cos\frac{13x+7x}{2} =2Sin3x Cos10x[/tex]