(x2+y2)2-xy(x+y)2＝x^4+y^4+2x^2y^2-x^3y-2x^2y^2-y^3x=x^3(x-y)-y^3(x-y)=(x-y)^2(x^2+xy+y^2)因为(x-y)^2>＝0主要看(x^2+xy+y^2)＝（x+y/2）^2+3y^2/4>=0所以(x2+y2)2-xy(x+y)2>=0当x=y等式成立所以(x2+y2)2>=xy(...