2x²y²-x的4次方-y的4次方
=-(x^4+y^4-2x^2y^2)
=-(x^2-y^2)^2
=-(x+y)^2(x-y)^2=-(x^2-y^2)^2=-(x+y)^2(x-y)^2没懂-(x^2-y^2)^2=-[(x+y)(x-y)]^2=-(x+y)^2(x-y)^2