答：
x^2-2xy+y^2=(x-y)^2
x^2+2xy+y^2=(x+y)^2
通常情况下它们不相等：x^2-2xy+y^2≠x^2+2xy+y^2
只有在xy=0的情况下他们才相等：x^2-2xy+y^2=x^2+2xy+y^2