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