Cos P/7 * cos 2*P/7 * cos 4*P/7

Решение:

домножи и разделим это выражение на 2sin (π/7)

2sin (π/7) * cos (π/7) * cos (2π/7) * cos (4π/7) / (2sin (π/7)) =

=sin (2π/7) * cos (2π/7) * cos (4π/7) / (2sin (π/7)) = 2sin (2π/7) * cos (2π/7) * cos (4π/7) / (4sin (π/7)) =

sin (4π/7) * cos (4π/7) / (4sin (π/7)) = 2sin (4π/7) * cos (4π/7) / (8sin (π/7)) = sin (8π/7) / (8sin (π/7)) = sin (π+π/7) / (8sin (π/7)) = - sib (π/7) / (8sin (π7) = - 1/8