sin21°cos189°-sin69°sin9°的值为

分类: 作业答案 • 2022-05-05 17:30:12

问题描述：

答

sin21°cos189°-sin69°sin9°
=-sin21°cos9°-cos21°sin9°
=-(sin21°cos9°+cos21°sin9°)
=-sin(21°+9°)
=-sin30°
=-1/2