原式==x^4+2x^2y+x^2y^2-4x^2y^2-4xy^3-y^4
=(x^2+xy)^2-(2xy+y^2)^2
=(x^2+xy+2xy+y^2)(x^2+xy-2xy-y^2)
=(x^2+3xy+y^2)(x^2-xy-y^2)