COSπ/15-COS2π/15-COS4π/15+COS7π/15= 2009年江西数学预赛的题要详解

囡囡Echo2022-10-04 11:39:542条回答

已提交，审核后显示！提交回复

共2条回复

300岁女孩共回答了14个问题 | 采纳率100%: 原式=(cos7π/15+cosπ/15)-(cos4π/15+cos2π/15)
=2cos4π/15cosπ/5-2cosπ/5cosπ/15
=2cosπ/5(cos4π/15-cosπ/15)
=-4cosπ/5sinπ/6sinπ/10
=-2cosπ/5cos2π/5
=(-2sinπ/5cosπ/5cos2π/5)/(sinπ/5)
=-(sin2π/5cos2π/5)/(sinπ/5)
=-(sin4π/5)(2sinπ/5)=-1/2; 1年前

练习快乐418 共回答了13个问题 | 采纳率: 和差化积; 1年前

相关推荐

大家在问