作业帮若cos（x+y)=4/5,cos(x-y)=-4/5,且90
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若cos（x+y)=4/5,cos(x-y)=-4/5,且90
若cos（x+y)=4/5,cos(x-y)=-4/5,且90
若cos（x+y)=4/5,cos(x-y)=-4/5,且90
cos(x+y)=4/5
270°＜x+y＜360°
所以sin(x+y)=-√[1-(4/5)^2]=-3/5
cos(x-y)=-4/5
90°＜x-y＜180°
所以sin(x-y)=√[1-(-4/5)^2]=3/5
所以cos2x=cos[(x+y)+(x-y)]=cos(x+y)cos(x-y)-sin(x+y)sin(x-y)=(4/5)*(-4/5)-(-3/5)*(3/5)=-7/25
cos2y=cos[(x+y)-(x-y)]=cos(x+y)cos(x-y)+sin(x+y)sin(x-y)=(4/5)*(-4/5)+(-3/5)*(3/5)=-1
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sin(x+y)=-3/5
sin(x-y)=3/5
cos2x=cos[(x+y)+(x-y)]=cos(x+y)cos(x-y)-sin(x+y)sin(x-y)=-16/25+9/25=-7/25
cos2y=cos[(x+y)-(x-y)]=cos(x+y)cos(x-y)+sin(x+y)sin(x-y)=-16/25-9/25=-1
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