cos12°-cos24°-cos48°+cos84°
问题描述：
cos12°-cos24°-cos48°+cos84°
答
cos12°-cos24°-cos48°+cos84° =cos12°+cos84°-cos24°-cos48°=2cos48°cos36°-2cos36°cos12°=2cos36°(cos48°-cos12°)=-2cos36°2sin30°sin18°=-2cos36°sin18°=-2cos36°cos72°=-2(√5+1)/4*[2(√5...