sinx+siny=3/5,cosx+cosy=4/5,求cos(x-y)

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sinx+siny=3/5,cosx+cosy=4/5,求cos(x-y)
sinx^2+cosx^2才＝1啊不要给我sinx^2+siny^2=1啊拜托大家帮帮忙
老大过程啊啊啊

cos(x-y)=cosx*cosy+sinx*siny
(sinx+siny)^2+(cosx+cosy)^2=1
sinx^2+cos^2+siny^2+cosy^2+2sinx*siny+2cosx*cosy=1
sinx*cosx+sinx*siny=-1/2=cos(x-y)

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