用到的公式:[cos(180°+A)=-cosA/sin(90°-A)=cosA/sin(A+B)=sinAcosB+cosAsinB]因为cos189°=cos(180°+9°)=-cos9°和sin69°=sin(90°-21°)=cos21°所以sin21°cos189°-sin69°sin9°=-sin21°cos9°-cos21°si...