Question

Докажите, что cos(п/7) * cos(4п/7) * cos(5п/7)=1/4

Answer 1

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