x^2+y^2-4x^2y^2=x^2+y^2-2x^2y^2-2x^2y^2=(x-y)^2-2x^2y^2=(x-y+√2xy)(x-y-√2xy)("√"表示根号,^表示次方)
(x-y+xy)(x-y-xy)