Bài 25.
Giả sử $a\geq b\geq c,$
$\frac{4(a^2+b^2+c^2)}{ab+bc+ca}+\sqrt{3}\sum_{cyc}\frac{a+2b}{\sqrt{a^2+2b^2}}-13$$=\sum_{cyc}\left ( \frac{1}{ab+bc+ca}-\frac{3}{(b+2c+\sqrt{3(b^2+2c^2)})\sqrt{3(b^2+2c^2)}} \right )(b-c)^2$$\geq \left ( \frac{1}{ab+bc+ca}-\frac{3}{(c+2a+\sqrt{3(c^2+2a^2)})\sqrt{3(c^2+2a^2)}} \right )(c-a)^2+\left ( \frac{1}{ab+bc+ca}-\frac{3}{(b+2c+\sqrt{3(b^2+2c^2)})\sqrt{3(b^2+2c^2)}} \right )(b-c)^2$$\geq \frac{1}{b^2}\left ( \frac{a^2}{ab+bc+ca}-\frac{3a^2}{(c+2a+\sqrt{3(c^2+2a^2)})\sqrt{3(c^2+2a^2)}}+\frac{b^2}{ab+bc+ca}-\frac{3b^2}{(b+2c+\sqrt{3(b^2+2c^2)})\sqrt{3(b^2+2c^2)}} \right )(b-c)^2$$\geq \frac{1}{b^2}\left ( \frac{a^2+b^2}{ab+bc+ca}-\frac{3a^2}{2(c+2a)^2}-\frac{3b^2}{2(b+2c)^2} \right )(b-c)^2$$\geq \frac{(8a^4b^2+8a^2b^4-15a^3b^3)+(19a^4c^2-19a^3b^2c)+(4a^4bc-4a^2bc^3)}{2b^2(ab+bc+ca)(c+2a)^2(b+2c)^2}\geq 0\blacksquare$
Bách coi lại bài này đâu thể giả sử $a\geq b\geq c$ và cái tô đỏ thứ 2 là chưa chắc lớn hơn