>>359

x-y=a、y-z=bで考えたら、

(x-y)^2 - (x-y)(y-z) + (y-z)^2
=a^2 - ab + b^2
=(a-b)^2
={(x-y)-(y-z)}^2
=(x-2y+z)^2

じゃダメなの？