x^4+2x^2y^2-2x^3y-2xy^3+y^4
=x²﹙x²＋y²﹚-2xy﹙x²+y²﹚＋y²﹙x²+y²﹚
=﹙x²+y²﹚﹙x²-2xy+y²﹚
=﹙x²+y²﹚﹙x-y﹚²
题中是不是漏掉了加y的四次方