已知x²-3xy+4x=3,y²+xy-4y+7=0,则xy-1/2(x²+y²)的值为
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![已知x²-3xy+4x=3,y²+xy-4y+7=0,则xy-1/2(x²+y²)的值为](/uploads/image/z/2095789-13-9.jpg?t=%E5%B7%B2%E7%9F%A5x%26%23178%3B-3xy%2B4x%3D3%2Cy%26%23178%3B%2Bxy-4y%2B7%3D0%2C%E5%88%99xy-1%2F2%28x%26%23178%3B%2By%26%23178%3B%29%E7%9A%84%E5%80%BC%E4%B8%BA)
已知x²-3xy+4x=3,y²+xy-4y+7=0,则xy-1/2(x²+y²)的值为
已知x²-3xy+4x=3,y²+xy-4y+7=0,则xy-1/2(x²+y²)的值为
已知x²-3xy+4x=3,y²+xy-4y+7=0,则xy-1/2(x²+y²)的值为
x²-3xy+4x=3,y²+xy-4y+7=0
相加得x²-2xy+y²+4(x-y)+4=0
(x-y)²+4(x-y)+4=0
(x-y+2)=0,得x-y=-2
xy-1/2(x²+y²)=-(1/2)(x-y)²=-(1/2)*4=-2