Có
$a^{3}+b^{^{3}}+c^3-3abc=\left ( a+b+c \right )\left ( a^2+b^2+c^2-ab-bc-ca\right )$$a^{3}+b^{^{3}}+c^3-3abc=\left ( a+b+c \right )\left ( a^2+b^2+c^2-ab-bc-ca\right )$
hay
$\frac{a^3+b^3+c^3}{abc}=3+\frac{(a+b+c)(a^2+b^2+c^2-ab-bc-ca)}{abc}=3+\left ( \frac{1}{ab}+\frac{1}{bc}+\frac{1}{ca} \right )(a^2+b^2+c^2-ab-bc-ca)\geq 3+\frac{9(a^2+b^2+c^2-ab-bc-ca)}{ab+bc+ca}=3+\frac{9(a^2+b^2+c^2)}{ab+bc+ca}-9$
ta chỉ can cm
$\frac{a^2+b^2+c^2}{ab+bc+ca}+\frac{ab+bc+ca}{a^2+b^2+c^2}\geq 2$(hiển nhiên đúng)
- perfectstrong và nthoangcute thích