<s>[INST] Using Rearrangement Inequality . 
Let $a,b,c\in\mathbf R^+$, such that $a+b+c=3$. Prove that $$\frac{a^2}{b+c}+\frac{b^2}{c+a}+\frac{c^2}{a+b}\ge\frac{a^2+b^2+c^2}{2}$$
$Hint$ : Use Rearrangement Inequality

My Work :-$\\$
Without Loss of Generality let's assume that $0\le a\le b\le c$ then we can infer that $c^2\ge b^2\ge a^2$ also $b+c\ge c+a \ge a+b$. Thus $\frac{1}{b+c}\le \frac{1}{c+a}\le \frac{1}{a+b}$. Hence by Rearrangement Inequality $\frac{a^2}{b+c}+\frac{b^2}{c+a}+\frac{c^2}{a+b}$ is the greatest permutation however I am not able to express $\frac{a^2+b^2+c^2}{2}$ as another permutation of the same, hence I require assistance.
Thank You
 [/INST] I'm not sure how to use Rearrangement Inequality but I can do it with tangent line method.

 Using $a+b+c=3$ we can rewrite OP as: $$\underbrace{{a^2\over 3-a}-{a^2\over 2}}_{f(a)} + {b^2\over 3-b}- {b^2\over 2}+{c^2\over 3-c} -{c^2\over 2} \geq 0$$ If you calculate a tangent at point $x=1$ on $f(x)$ we get $y={1\over 4} (x-1)$ and it is not hard to see that for $x\in (0,3)$ we have $$f(x)\geq {1\over 4}(x-1)$$


 Using this we get $$f(a) +f(b)+f(c)\geq {1\over 4}(a-1)+ {1\over 4}(b-1)+{1\over 4}(c-1)=0$$

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