\frac{y^2+x^2-2xy}{xz-yz+ty-xt}=\frac{(x-y)^2}{(xz-yz)+(ty-xt)}=\frac{(x-y)^2}{z(x-y)+t(y-x)}=\frac{(x-y)^2}{(z+t)(x-y)}=\frac{(x-y)}{(z+t)}
(x-y)^2=(y-x)^2
