问题
填空题
求值：sin21°cos81°-sin69°cos9°=______．
答案
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
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