числитель =
=1 + Cos50° -Cos25° = 1 + 2Cos²25° -1 -Cos25° = 2Cos²25° -Cos25° =
=Cos25°(2Cos25° -1)
знаменатель = 2Sin25°Cos25° -Sin25° = Sin25°(2Cos25° -1)
Сама дробь = Ctg25°
Наш пример:
Ctg25° - tg65° = tg65° - tg65° = 0 (Сtg65° = Ctg(90° -25°) = tg25° )