[Toán 8] Khó

E

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Từ GT \Rightarrow $a=-b-c$

$ab+2bc+3ca=b(-b-c)+2bc+3c(-b-c)=-b^2-2bc-3c^2 \le 0$

\Leftrightarrow $b^2+2bc+3c^2 \ge 0$

\Leftrightarrow $(b+c)^2+2c^2 \ge 0$ (luôn đúng)
 
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