-cos (π/7)*cos(4π/7)*cos(2π/7) равно?

-cos(π/7)*cos(2π/7)*cos(4π/7)==-sin(π/7)*cos(π/7)*cos(2π/7)*cos(4π/7)/sin(π/7)==-sin(2π/7)*cos(2π/7)*cos(4π/7)/[2sin(π/7)]==-sin(4π/7)*cos(4π/7)/[4sin(π/7)]=-sin(8π/7)/[-8sin(π/7)]==-sin(π-π/7)/[8sin(π/7)]=-sin(π/7)/[8sin(π/7)]=-1/8