cos20°-cos40°-cos80°=_．
问题描述：
cos20°-cos40°-cos80°=______．
答
cos20°-cos40°-cos80°=cos20°-cos（60°-20°）-cos（60°+20°）=cos20°-（cos60°cos20°+sin60°sin20°）-（cos60°cos20°-sin60°sin20°）=cos20°-cos60°cos20°-sin60°sin20°-cos60°cos20°+sin60...