求证cos2π/7+cosπ/3=cosπ/7+cos3π/7

来源：学生作业帮编辑：作业帮分类：数学作业时间：2024/05/16 19:27:22

cosπ/7-cos2π/7+cos3π/7
=cosπ/7+cos3π/7+cos5π/7
=(2sinπ/7*cosπ/7+2sinπ/7*cos3π/7+2sinπ/7*cos5π/7)/(2sinπ/7)
=[sin2π/7+(sin4π/7-sin2π/7)+(sin6π/7-sin4π/7)]/(2sinπ/7)
=(sin6π/7)/(2sinπ/7)
=(sinπ/7)/(2sin/π7)
=1/2
=cosπ/3

求证cos2π/7+cosπ/3=cosπ/7+cos3π/7 已知(sinα+sin2α+sin3α)/(cosα+cos2α+cos3α)=﹣4/tanα,且π＜2α＜3π/2,求已知cos2α=-7/25,2α属于(π,3π/2).求sinα,cosα COSπ/5乘以COS2π/5 求COSπ/5*COS2π/5 化简：cosαcos2αcos3α 已知sinα与sinβ分别是sinθ与cosθ的等差中项与等比数列的中项,求证:2cos2α=cos2β=2{cos(π 若sin(π/4+α)=sinθ+cosθ,2sin^2β=sin2θ,求证：sin2θ+2cos2β=3 sinπ.cosπ.sin2π.cos2π分别是多少 cosπ/5·cos2π/5等于多少? （cosπ/5）（cos2π/5)的值等于 cosπ/5 乘以cos2π/5等于多少