$(x-y)^2(y-z)^2+(y-z)^2(z-x)^2+(z-x)^2(x-y)^2$
$={[(x-y)(y-z)]^2+[(y-z)(z-x)]^2+[(z-x)(x-y)]^2+2(x-y)(y-z)(z-x)(x-y)+(y-z)(z-x)(x-y)(y-z)+(z-x)(x-y)(y-z)(z-x)}-2(x-y)(y-z)(z-x)(x-y)+(y-z)(z-x)(x-y)(y-z)+(z-x)(x-y)(y-z)(z-x)$
$=[(x-y)(y-z)+(y-z)(z-x)+(z-x)(x-y)]^2-2(x-y)(y-z)(z-x)(x-y+y-z+z-x)$
$=[(x-y)(y-z)+(y-z)(z-x)+(z-x)(x-y)]^2$
$=(x^2+y^2+z^2-xy-yz-xz)^2$