<s>[INST] prove that $\sum_{cyc}\frac{a^2}{b} +\frac{3n}{a^2+b^2+c^2}\ge 3+n$ prove that
$$\sum_{cyc}\frac{a^2}{b} +\frac{3n}{a^2+b^2+c^2}\ge 3+n$$
where $a,b,c>0$ and $n\ge0,n\le 3$,$a+b+c=ab+bc+ca$
My try: by given condition $a+b+c=ab+bc+ca$
we have $a+b+c\le a^2+b^2+c^2$
also using titu's lemma
$$\sum_{cyc}\frac{a^2}{b} +\frac{3n}{a^2+b^2+c^2}\ge a+b+c +\frac{3n}{a^2+b^2+c^2}$$
or
$$\sum_{cyc}\frac{a^2}{b} +\frac{3n}{a^2+b^2+c^2}\ge ab+bc+ca +\frac{3n}{a^2+b^2+c^2}$$
i dont know what to do next. Any ideas preferably using am-gm.
source Samin Riasat Basics in Olympiad ineq.
 [/INST] We only need to prove this inequality for $n=3.$ Write the inequality as homogeneous form$:$
$${\dfrac {{a}^{2}}{b}}+{\dfrac {{b}^{2}}{c}}+{\dfrac {{c}^{2}}{a}}+{
\dfrac { 9\left( ab+bc+ac \right) ^{3}}{ \left( {a}^{2}+{b}^{2}+{c}^{2}
 \right)  \left( a+b+c \right) ^{3}}}\geqslant {\dfrac {6(ab+bc+ac)}{a+b+c}}\quad (\text{1})$$
Since the inequality $(\text{1})$ is homogeneous$,$ we may assume $a+b+c=1.$
Let $ab+bc+ca=\frac{1-t^2}{3} \quad (t\in [\,0,1\,]),abc=r.$
We can prove$:$ $${\dfrac {{a}^{2}}{b}}+{\dfrac {{b}^{2}}{c}}+{\dfrac {{c}^{2}}{a}}\geqslant {\dfrac {3\,{t}^{4}-4\,{t}^{3}-5\,{t}^{2}-2\,t-1}{ \left( t-1 \right) \left( 1+2\,t \right)  \left( t+1 \right) }} (\text{2}),$$
You can prove this inequality by yourself, I will post my proof for it in another question.
Therefore we need to prove$:$ $${\frac {3\,{t}^{4}-4\,{t}^{3}-5\,{t}^{2}-2\,t-1}{ \left( t-1 \right)  \left( 1+2\,t \right)  \left( t+1 \right) }}+2\,({t}^{2}-1)-{ \frac { \left( t-1 \right) ^{3} \left( t+1 \right) ^{3}}{1+2\,{t}^{2}} }\geqslant 0.$$
I think now, you can prove it very easy! And the inequality $(\text{2})$ is strong!
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