若sinθ:sinθ/2=8:5,则cosθ等于
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若sinθ:sinθ/2=8:5,则cosθ等于
![若sinθ:sinθ/2=8:5,则cosθ等于](/uploads/image/z/8770847-23-7.jpg?t=%E8%8B%A5sin%CE%B8%EF%BC%9Asin%CE%B8%2F2%3D8%EF%BC%9A5%2C%E5%88%99cos%CE%B8%E7%AD%89%E4%BA%8E)
sinθ:sinθ/2=2sinθ/2*cosθ/2:sinθ/2=2cosθ/2=8/5
cosθ/2=4/5
所以cosθ=cos(2*θ/2)=2(cosθ/2)^2-1=2*16/25-1=7/25
cosθ/2=4/5
所以cosθ=cos(2*θ/2)=2(cosθ/2)^2-1=2*16/25-1=7/25
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