(x+y)^2=9,x^2+y^2+2xy=9,x^2+y^2=9-2=7;(x-Y)^2=x^2+y^2-2xy=7-2=5,x-y=正负根号5，（x^2+y^2)^2=7^2,x^4+y^4+2x^2y^2=49,x^4+y^4=49-2=47
