[tex]P\leqslant \sqrt{3\left (\frac{1}{5a^2+2ab+2b^2}+\frac{1}{5b^2+2cb+2c^2}+\frac{1}{5c^2+2ac+2a^2} \right )}\leqslant \sqrt{3\left ( \frac{1}{81}\left ( \sum \frac{7}{a^2}+\sum{\frac{2}{ab}} \right ) \right )}\leqslant \sqrt{\frac{1}{27}\left (\sum{\frac{9}{a^2}} \right )}=\frac{1}{\sqrt{3}}[/tex]
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[tex]P\leqslant \sqrt{3\left (\frac{1}{5a^2+2ab+2b^2}+\frac{1}{5b^2+2cb+2c^2}+\frac{1}{5c^2+2ac+2a^2} \right )}\leqslant \sqrt{3\left ( \frac{1}{81}\left ( \sum \frac{7}{a^2}+\sum{\frac{2}{ab}} \right ) \right )}\leqslant \sqrt{\frac{1}{27}\left (\sum{\frac{9}{a^2}} \right )}=\frac{1}{\sqrt{3}}[/tex]
dùng bunhia với cô si dạng phân thức