cos P7 * cos 2*P7 * cos 4*P7

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