=(-1/2)【cos(π/3)-cos(4π/18)】sin(7π/18)=(-1/2)【(1/2)-cos(4π/18)】sin(7π/18)=(-1/4)sin(7π/18)-(-1/2)cos(4π/18)sin(7π/18)=(-1/4)sin(7π/18)-(-1/4)【sin(11π/18...
=(-1/2)【cos(π/3)-cos(4π/18)】sin(7π/18)=(-1/2)【(1/2)-cos(4π/18)】sin(7π/18)=(-1/4)sin(7π/18)-(-1/2)cos(4π/18)sin(7π/18)=(-1/4)sin(7π/18)-(-1/4)【sin(11π/18...