作业帮cos140°cos20°-sin220°sin20°
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cos140°cos20°-sin220°sin20°
cos140°cos20°-sin220°sin20°
cos140°cos20°-sin220°sin20°
cos140°cos20°-sin220°sin20°=cos140°cos20°-sin(360°-140°)sin20°
=cos140°cos20°+sin140°sin20°
=cos120°=-1/2
220换算成360-140就可以了，然后用二倍角公式
cos140°cos20°-sin220°sin20°
cos140°cos20°-sin220°sin20°
cos140°cos20°-sin220°sin20°
若cos140°=a,则sin220的值为?
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