问题描述: 化简cosπ/7*cos2π/7*cos4π/7 提示:据分析应该用倍角公式. 1个回答 分类:数学 2014-11-29 问题解答: 我来补答 cos(π/7)*cos(2π/7)*cos(4π/7)=[2sin(π/7)cos(π/7)*cos(2π/7)*cos(4π/7)]/2sin(π/7)=[sin(2π/7)*cos(2π/7)*cos(4π/7)]/2sin(π/7)=[2sin(2π/7)*cos(2π/7)*cos(4π/7)]/4sin(π/7)=[sin(4π/7)*cos(4π/7)]/4sin(π/7)=[2sin(4π/7)*cos(4π/7)]/8sin(π/7)=sin(8π/7)/8sin(π/7) (因为sin(8π/7)=-sin(π/7))=-1/8 展开全文阅读