x^2+y^2=15,x+y=7,x>y,求x-y
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x^2+y^2=15,x+y=7,x>y,求x-y
![x^2+y^2=15,x+y=7,x>y,求x-y](/uploads/image/z/20005456-40-6.jpg?t=x%5E2%2By%5E2%3D15%2Cx%2By%3D7%2Cx%EF%BC%9Ey%2C%E6%B1%82x-y)
(x-y)^2=(x^2+y^2)-2xy
=(x^2+y^2)-[(x+y)^2-(x^2+y^2)]
=2(x^2+y^2)-(x+y)^2
=2*15-7^2=-19
=(x^2+y^2)-[(x+y)^2-(x^2+y^2)]
=2(x^2+y^2)-(x+y)^2
=2*15-7^2=-19
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