<s>[INST] If $a+b+c=1$, find the minimum of $\frac{4+3abc}{ab+bc+ac}$ I came to ask this because I am really stuck at this problem. I have tried everything from arithmetic mean, geometric mean and harmonic mean. Also, I have tried playing with the variables and such, but it got me to nowhere.
If $a+b+c=1$; $a,b,c$ nonnegative, calculate the minimum of 
$$\frac{4+3abc}{ab+bc+ac}$$
All I've got so far is:
$$\frac{3abc}{ab+bc+ac} \le \frac{1}{3}$$
But this is obviously on the wrong side of the inequality.
Also, I think that
$$\frac{1}{ab+bc+ac}\ge3$$
But I haven't been able to prove it.
Playing with the most possible and obvious values, one could think that the answer is 37/3, but the excercise is about proving it. Any help and little hints are greatly apprecieated.
 [/INST] Proof
Denote $u = ab + bc + ca$, we have the following two results
(1) $0 \le 3 u \le 1$,
and
(2) $9abc \ge 4u - 1$.
Then,
$$
3abc+4
= \frac{9abc+12}{3}
\ge \frac{4u + 11}{3}
\ge \frac{4u + 11\times3u}{3} = \frac{37}{3} u.
$$
which is the desired result.
Lemma 1
To show (1), we expand
$$(a-b)^2 + (b-c)^2 + (c-a)^2 \ge 0$$
which gives
$$(a + b + c)^2 - 3 (ab+bc+ca) \ge 0$$
which means $1 \ge 3u$.  The $u \ge 0$ part is obvious for $a$, $b$, and $c$ are nonnegative.
Lemma 2
For (2), we can use Schur's inequality
$$a^3+b^3+c^3+3abc
\ge
a^2(b+c)+b^2(c+a)+c^2(a+b).$$
Since $b + c = 1 - a$, etc, we get
$$2(a^3+b^3+c^3)+3abc \ge a^2+b^2+c^2.$$
Using
$$
a^2 + b^2 + c^2 = (a+b+c)^2 - 2 (ab+bc+ca),
$$
and 
$$
a^3 + b^3 +c^3 - 3abc
= (a + b + c)^3 - 3 (a+b+c) (a b + b c + c a).
$$
we get
$$
2(1-3u)+9abc \ge 1-2u,
$$
which is Lemma 2.
Generalization
Generally, we can change the coefficients a bit,
$$
\frac{ 9 p \, a b c + q + p r + 3 q r } { a b + b c + c a + r }
\ge
p + 3q,
$$
and the problem is the special case of $p = \frac{1}{3}, q = 4, r = 0$.
This problem inspires another one, and the use of Schur's inequality is suggested by Dylan.
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