（x-z)的平方-4（y-z)(x-y)
=[(x-y)+(y-z)]^2-4(y-z)(x-y)
=(x-y)^2+2(x-y)(y-z)+(y-z)^2-4(y-z)(x-y)
=(x-y)^2-2(x-y)(y-z)+(y-z)^2
=[(x-y)-(y-z)]^2
=(x-2y+z)^2