求值：sin21°cos81°-sin69°cos9°=_．
问题描述：
求值：sin21°cos81°-sin69°cos9°=______．
答
sin21°cos81°-sin69°cos9°=sin21°cos（90°-9°）-sin（90°-21°）cos9°
=sin21°sin9°-cos21°cos9°=-（cos21°cos9°-sin21°sin9°）=-cos（21°+9°）=-cos30°=-
3
2
故答案为：-
3
2