求证:对于任何实数x,y都有x的四次方+y的四次方≥1/2xy(x+y)平方

原式化为：
2x4+2y4-x3y-2x2y2-xy3>=0
该式
=2(x4-2x2y2+y4)+2x2y2-x3y-xy3
=2(x2-y2)2-xy(x-y)2
=2(x-y)2[(x+y)2-xy]
=2(x-y)2[x2+y2+xy]
x2+y2+xy>=0;
(x-y)2>=0;
所以证题成立