求2和3题,大哥大姐们帮帮忙
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求2和3题,大哥大姐们帮帮忙
![](http://img.wesiedu.com/upload/7/8b/78b50c7b800a2d4624a0422382c1e5bf.jpg)
![](http://img.wesiedu.com/upload/7/8b/78b50c7b800a2d4624a0422382c1e5bf.jpg)
![求2和3题,大哥大姐们帮帮忙](/uploads/image/z/16522292-20-2.jpg?t=%E6%B1%822%E5%92%8C3%E9%A2%98%2C%E5%A4%A7%E5%93%A5%E5%A4%A7%E5%A7%90%E4%BB%AC%E5%B8%AE%E5%B8%AE%E5%BF%99)
设θ=x+y,Ψ=x-y;x=(θ+Ψ)/2,y=(θ-Ψ)/2
cosθ+cosΨ=cos(x+y)+cos(x-y)=cosxcosy-sinxsiny+cosxcosy+sinxsiny=2cosxcosy=2cos(θ+Ψ)/2 cos(θ+Ψ)/2
cosθ-cosΨ=cos(x+y)-cos(x-y)=cosxcosy-sinxsiny-cosxcosy-sinxsiny=-2sinxsiny=-2sin(θ+Ψ)/2 sin(θ+Ψ)/2
cosθ+cosΨ=cos(x+y)+cos(x-y)=cosxcosy-sinxsiny+cosxcosy+sinxsiny=2cosxcosy=2cos(θ+Ψ)/2 cos(θ+Ψ)/2
cosθ-cosΨ=cos(x+y)-cos(x-y)=cosxcosy-sinxsiny-cosxcosy-sinxsiny=-2sinxsiny=-2sin(θ+Ψ)/2 sin(θ+Ψ)/2