Cách khác ;Cho 3 số dương a, b, c thỏa mãn ab + bc + ca = 3. CMR : $\sum \frac{1}{1+a^2(b+c)}\leq \frac{1}{abc}$
Ta có : $ab+bc+ca\geq 3\sqrt[3]{(abc)^2}\Rightarrow abc\leq 1$
$\Rightarrow \sum \frac{1}{1+a^2(b+c)}\leq \sum \frac{1}{abc+a^2(b+c)}= \sum \frac{1}{a(bc+ac+ac)}= \sum \frac{1}{3a}$
$= \frac{1}{3a}+\frac{1}{3b}+\frac{1}{3c}= \frac{ab+bc+ca}{3abc}= \frac{1}{abc}$