Đáp án:
`x^4+y^4>=xy(x^2+y^2)`
`<=>x^4+y^4>=x^3y+xy^3`
`<=>x^4-x^3y+y^4-xy^3>=0`
`<=>x^3(x-y)-y^3(x-y)>=0`
`<=>(x-y)(x^3-y^3)>=0`
`<=>(x-y)(x-y)(x^2+xy+y^2)>=0`
`<=>(x-y)^2(x^2+xy+y^2)>=0`
`<=>(x-y)^2[(x+1/2y)^2+3/4y^2]>=0` luôn đúng
Dấu "=" xảy ra khi `x=y`