[imath]\Leftrightarrow 2(a^4+b^4) \geq 2(a^3+b^3)\Leftrightarrow2(a^4+b^4) \geq (a+b)(a^3+b^3)[/imath]
[imath]\Leftrightarrow a^4-a^3b-ab^3+b^4 \geq 0 \Leftrightarrow a^3(a-b)-b^3(a-b) \geq 0[/imath]
[imath]\Leftrightarrow (a-b)(a^3-b^3) \geq 0 \Leftrightarrow (a-b)^2(a^2+ab+b^2) \geq 0[/imath] (luôn đúng)