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Algebra Difficulty 6.9 National olympiad Prove it

Problem 22 Let a,b,ca, b, c be positive real numbers such that a+b+c=3a+b+c=3. Prove that
a2b+b2c+c2a+2(ab2+bc2+ca2)63a^{2} b+b^{2} c+c^{2} a+2\left(a b^{2}+b c^{2}+c a^{2}\right) \leq 6 \sqrt{3}

Solution

Solution: The inequality equivalent to
2cyca2b+4cycab21233syma2(b+c)+(cycab2cyca2b)1233cyca2(b+c)+(ab)(bc)(ca)123\begin{array}{c} \Leftrightarrow 2 \sum_{cyc} a^{2} b+4 \sum_{cyc} a b^{2} \leq 12 \sqrt{3} \\ \Leftrightarrow 3 \sum_{sym} a^{2}(b+c)+\left(\sum_{cyc} a b^{2}-\sum_{cyc} a^{2} b\right) \leq 12 \sqrt{3} \\ \Leftrightarrow 3 \sum_{cyc} a^{2}(b+c)+(a-b)(b-c)(c-a) \leq 12 \sqrt{3} \end{array}

We only need to prove the inequality in the case (ab)(bc)(ca)0(a-b)(b-c)(c-a) \geq 0.
3sym a2(b+c)+(ab)2(bc)2(ca)21233(pq3r)+p2q2+18pqr27r24q34p3r123p2q2+18pqr27r24q34p3r(1233pq+9r)2f(r)=108r2+(4p372pq+2163)r+4q3+8p2q2723pq+4320\begin{array}{l} 3 \sum_{\text {sym }} a^{2}(b+c)+\sqrt{(a-b)^{2}(b-c)^{2}(c-a)^{2}} \leq 12 \sqrt{3} \\ \Leftrightarrow 3(p q-3 r)+\sqrt{p^{2} q^{2}+18 p q r-27 r^{2}-4 q^{3}-4 p^{3} r} \leq 12 \sqrt{3} \\ \Leftrightarrow p^{2} q^{2}+18 p q r-27 r^{2}-4 q^{3}-4 p^{3} r \leq(12 \sqrt{3}-3 p q+9 r)^{2} \\ \Leftrightarrow f(r)=108 r^{2}+\left(4 p^{3}-72 p q+216 \sqrt{3}\right) r+4 q^{3}+8 p^{2} q^{2}-72 \sqrt{3} p q+432 \geq 0 \end{array}

Putting rct=216q1082163108r_{ct}=\frac{216 q-108-216 \sqrt{3}}{108}
Let two cases
1) 0q2163+108216rct00 \leq q \leq \frac{216 \sqrt{3}+108}{216} \Rightarrow r_{ct} \leq 0
f(0)=4(q+12+63)(q+33)20f(0)=4(q+12+6 \sqrt{3})(q+3-\sqrt{3})^{2} \geq 0
2) 2163+108216q3rct0\frac{216 \sqrt{3}+108}{216} \leq q \leq 3 \Rightarrow r_{ct} \geq 0
f(rct)=4q336q2+108q+8110830,q[3+12;3]f\left(r_{ct}\right)=4 q^{3}-36 q^{2}+108 q+81-108 \sqrt{3} \geq 0, \forall q \in\left[\sqrt{3}+\frac{1}{2} ; 3\right]

So f(r)rr0f(r) \geq r \forall r \geq 0. The solution is end.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.