已知cosx+cosy=1/2,sinx-siny=1/3,求cos(x+y)
人气：212 ℃ 时间：2019-10-02 21:12:58
解答
有cos(x+y)=cosxcosy-sinxsiny又将题目所给式子左右平方,在两个式子左右对应相加为：cos^2x+cos^2y+2cosxcsoy+sin^2x+sin^2y-2sinxsiny=13/36由cos^2x+sin^2x=1 cos^2y+sin^2y=1 可得 1+2cosxcsoy+1-2sinxsiny=13/36 2cosxcsoy-2sinxsin=59/36cosxcosy-sinxsiny=59/72
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