x的四次方+y的四次方-3乘x的平方乘y的平方==x^4-2x^2y^2+y^4-x^2y^2=(x^2-y^2)^2-x^2y^2=(x^2+xy-y^2)(x^2-xy-y^2)
x^4-2x^2y^2+4y^4-x^2y^2=（x^2-y^2）^2-x^2y^2=（x^2-y^2+xy）（x^2-y^2-xy）
=(x^2+xy-y^2)(x^2-xy-y^2)
