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