Докажите, что cos(п/7) * cos(4п/7) * cos(5п/7)=1/4
Cos(5π/7) = -cos(2π/7) - 8sin (π/7) * cos (π/7) * cos (2π/7) * cos (4π/7) = - sin (8π/7) - sin (8π/7) / 8sin (π/7) = 1/8