证:(x+y+z)^2=x^2+y^2+z^2+2(xy+yz+zx)=0x^2+y^2+z^2=-2(xy+yz+zx)|1／x+1／y+1／z=|(xy+yz+zx)／xyz|=|(x^2+y^2+z^2)／2xyz|
