求值:sin21°cos81°-sin69°cos9°=______.-数学

题目简介

求值:sin21°cos81°-sin69°cos9°=______.-数学

题目详情

求值:sin21°cos81°-sin69°cos9°=______.
题型:填空题难度:中档来源:不详

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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