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Algebra Difficulty 7.3 National olympiad, round 2 Prove it Vietnam

Let aa, bb and cc be non-negative real numbers such that
2(a2+b2+c2)+3(ab+bc+ca)=5(a+b+c). 2(a^2 + b^2 + c^2) + 3(ab + bc + ca) = 5(a + b + c).
Prove that 4(a2+b2+c2)+2(ab+bc+ca)+7abc254(a^2 + b^2 + c^2) + 2(ab + bc + ca) + 7abc \le 25.

Solution

Let p=a+b+cp = a + b + c, q=ab+bc+caq = ab + bc + ca and r=abcr = abc, we have
2(p22q)+3q=5p or 2p2=5p+q.(1) 2(p^2 - 2q) + 3q = 5p \text{ or } 2p^2 = 5p + q. \quad (1)

We need to prove that 4(p22q)+2q+7r254(p^2 - 2q) + 2q + 7r \le 25 or 4p2+7r25+6q4p^2 + 7r \le 25 + 6q.
Since q=p25pq = p^2 - 5p, the inequality is equivalent to:
7r+30p8p2+25. 7r + 30p \le 8p^2 + 25.
Notice that (xy+yz+zx)23xyz(x+y+z)(xy + yz + zx)^2 \ge 3xyz(x + y + z) implies q23prq^2 \ge 3pr. We distinguish two cases regarding the value of pp.

* If p=0p = 0 then a=b=c=0a = b = c = 0, so the first inequality is true.

* If p>0p > 0 then rq23pr \le \frac{q^2}{3p}, so we have to prove that
7q23p+30p8p2+257(2p25p)2+90p224p3+75pp(p3)(2p5)(14p5)0.(2) \begin{aligned} & 7\frac{q^2}{3p} + 30p \le 8p^2 + 25 \\ & 7(2p^2 - 5p)^2 + 90p^2 \le 24p^3 + 75p \\ & p(p-3)(2p-5)(14p-5) \le 0. \end{aligned} \quad (2)
On the other hand, 2p25p=qp232p^2 - 5p = q \le \frac{p^2}{3} implies 52p3\frac{5}{2} \le p \le 3, so inequality (2) is true. Therefore, the inequality (1) is true.

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