<s>[INST] MOP 2011 inequality If $a,b,c$ are positive integers prove that 

$\sqrt{(a^2-ab+b^2)} +\sqrt{(b^2+c^2-bc)} +\sqrt{(a^2+c^2-ac)} +9(abc)^{1/3} \le 4(a+b+c)$

My attempt:
I tried to split inequality and prove it bit by bit
$9(abc)^{1/3} \le 3(a+b+c)$
It suffices to prove that 
$\sqrt{(a^2-ab+b^2)} +\sqrt{(b^2+c^2-bc)} +\sqrt{(a^2+c^2-ac)} \le a+b+c$
After trying out few values I realized it is wrong.So I decided to tackle it all at your once. I tried few well known inequalities and played around but nothing came useful out of it. Any help??
 [/INST] As we can see that the term $9(abc)^{\frac{1}{3}}$ is difficult to deal with, we consider the following inequality $$-\sqrt{(a^2-ab+b^2)} -\sqrt{(b^2+c^2-bc)} -\sqrt{(a^2+c^2-ac)} +4(a+b+c)\ge 9(abc)^{1/3}.$$
Then we consider $$\frac{a+b}{2}-\sqrt{(a^2-ab+b^2)}=\frac{(\frac{a+b}{2})^2-(a^2-ab+b^2)}{\frac{a+b}{2}+\sqrt{(a^2-ab+b^2)}},$$the numerator is non-positive, and the denominator $$\frac{a+b}{2}+\sqrt{(a^2-ab+b^2)}\ge \frac{a+b}{2}+\frac{a+b}{2}.$$So we have $$\frac{(\frac{a+b}{2})^2-(a^2-ab+b^2)}{\frac{a+b}{2}+\sqrt{(a^2-ab+b^2)}}\ge \frac{(\frac{a+b}{2})^2-(a^2-ab+b^2)}{a+b}=\frac{3ab}{a+b}-\frac{3}{4}(a+b).$$ Hence $$-\sqrt{(a^2-ab+b^2)} -\sqrt{(b^2+c^2-bc)} -\sqrt{(a^2+c^2-ac)} +4(a+b+c)\ge \sum_{cyc} \frac{3ab}{a+b}+\frac{3}{2}\sum a.$$Then you just need to use AM-GM inequality to prove it is greater than $9(abc)^{1/3}$.
One of my schoolmate of lower grade also finds a solution(before me).With the permission of him, I post his solution as follows:$$2(a+b)-\sqrt{a^2+b^2-ab}=\frac{a+b}{2}+\frac{1}{2}\sqrt{a^2+b^2-ab}+\frac{3(a+b)}{2}-\frac{3}{2}\sqrt{a^2+b^2-ab}$$$$=\frac{a+b+\sqrt{a^2+b^2-ab}}{2}+\frac{3}{2}\frac{3ab}{a+b+\sqrt{a^2+b^2-ab}}\ge 3\sqrt{ab}$$
So sum up and use AM-GM, we can finish the proof.
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