x+y-z=a
(x+y-z)^2=a^2
即：x^2+y^2+z^2+2xy-2xz-2yz=a^2
x^2+y^2+z^2+2(xy-xz-yz)=a^2
x^2+y^2+z^2+2b=a^2
x^2+y^2+z^2=a^2-2b
令t=-z
x+y+t=a,
xy+yt+tx=b
x^2+y^2+t^2=a^2-2b=x^2+y^2+z^2
x^2+y^2+z^2+2xy+2yz+2xz-2xy-2yz-2xz
=(x+y+z)^2-2(xy+yz+xz)
=a^2-2b
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