-(x-y)^2 - 2(y-x)^2 - 5(x-y)^2 +8 (x-y)^2
= -(x-y)^2 - 2(x-y)^2 - 5(x-y)^2 +8 (x-y)^2
= (-1-2-5+8)(x-y)^2
= 0