(x+y+z)^2=x^2+y^2+z^2+2xy+2yz+2xz又因为x^2+y^2>=2xy所以(x+y+z)^2=x^2+y^2+z^2+2xy+2yz+2xz>=4xy+z方+2yz+2xz=8因此x+y+z>=2*根号2最小值就=2*根号2
