查人人中国名人网张星臣:高一数学
来源：百度文库 编辑：查人人中国名人网 时间：2024/04/29 03:13:25
cos2π/7*cos4π/7*cos6π/7 答案是1/8,过程是?
cos6π/7=-cosπ/7
所以原式=-cosπ/7cos2π/7cos4π/7
=-cosπ/7cos2π/7cos4π/7*sinπ/7/(sinπ/7)
=-1/2sin2π/7cos2π/7cos4π/7/(sinπ/7)
=-1/4sin4π/7cos4π/7/(sinπ/7)
-1/8sin8π/7/(sinπ/7)
=1/8sinπ/7/(sinπ/7)
=1/8