已知COSa+COSb+COSy=0,SINa+SINb+SINy=0,求COS(b-y)的值
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已知COSa+COSb+COSy=0,SINa+SINb+SINy=0,求COS(b-y)的值
![已知COSa+COSb+COSy=0,SINa+SINb+SINy=0,求COS(b-y)的值](/uploads/image/z/14475535-7-5.jpg?t=%E5%B7%B2%E7%9F%A5COSa%2BCOSb%2BCOSy%3D0%2CSINa%2BSINb%2BSINy%3D0%2C%E6%B1%82COS%28b-y%29%E7%9A%84%E5%80%BC)
COS(b-y)=COSbCOSy+SINbSIBy COSa+COSb+COSy=0,(COSb+COSy)=COSa COSb+2COSbCOSy+COSy=COSa COSbCOSy=(COSa-COSb-COSy)/2 同理:SINbSINy=(SINa-SINb-SINy)/2 则COS(b-y)=COSbCOSy+SINbSIBy=(COSa-COSb-COSy)/2+(SINa-SINb-SINy)/2 =(COSa+SINa-COSb-SINb-COSy-SINy)/2 =(1-1-1)/2 =-1/2
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