cos(2π/7)cos(4π/7)cos(6π/7)=cos(2π/7)cos(4π/7)[-cos(π-6π/7)]=-cos(π/7)cos(2π/7)cos(4π/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)]=-2...