Умножаешь и делишь на 8sin(π/7).
Затем собираешь синусы двойных углов.
-8sin(π/7)*cos(π/7)*cos(2π/7)*cos(4π/7)/(8sin(π/7))=
-4sin(2π/7)*cos(2π/7)*cos(4π/7)/(8sin(π/7))=
-2sin(4π/7)*cos(4π/7)/(8sin(π/7))=
-sin(8π/7)/(8sin(π/7))=-(-sin(π/7))/(8sin(π/7)) = 1/8