Maths Olympiad Prep

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Algebra Difficulty 5.5 AIME, harder Prove it JBMO

Problem:
Let aa, bb, cc be positive real numbers with abc=1abc=1. Prove the inequality:
2a2+1ab+1a+1+2b2+1bc+1b+1+2c2+1ca+1c+13 \frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1}+\frac{2 b^{2}+\frac{1}{b}}{c+\frac{1}{b}+1}+\frac{2 c^{2}+\frac{1}{c}}{a+\frac{1}{c}+1} \geq 3

Solution

Solution:
By AMGMAM-GM we have 2x2+1x=x2+x2+1x3x4x3=3x2x^{2}+\frac{1}{x}=x^{2}+x^{2}+\frac{1}{x} \geq 3 \sqrt[3]{\frac{x^{4}}{x}}=3x for all x>0x>0, so we have:

cyc2a2+1ab+1a+1cyc3a1+b+bc=3(cyca21+a+ab)3(a+b+c)23+a+b+c+ab+bc+ca\sum_{\text{cyc}} \frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1} \geq \sum_{cyc} \frac{3a}{1+b+bc}=3\left(\sum_{cyc} \frac{a^{2}}{1+a+ab}\right) \geq \frac{3(a+b+c)^{2}}{3+a+b+c+ab+bc+ca}.

By AMGMAM-GM we have ab+bc+ca3ab+bc+ca \geq 3 and a+b+c3a+b+c \geq 3. But 3(a2+b2+c2)(a+b+c)23(a+b+c)3\left(a^{2}+b^{2}+c^{2}\right) \geq (a+b+c)^{2} \geq 3(a+b+c). So (a+b+c)2=a2+b2+c2+2ab+2bc+2ca3+a+b+c+ab+bc+ca(a+b+c)^{2}=a^{2}+b^{2}+c^{2}+2ab+2bc+2ca \geq 3+a+b+c+ab+bc+ca. Hence cyc2a2+1ab+1a+13(a+b+c)23+a+b+c+ab+bc+ca3(a+b+c)2(a+b+c)2=3\sum_{cyc} \frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1} \geq \frac{3(a+b+c)^{2}}{3+a+b+c+ab+bc+ca} \geq \frac{3(a+b+c)^{2}}{(a+b+c)^{2}}=3.

Denote a=yxa=\frac{y}{x}, b=zyb=\frac{z}{y} and c=xzc=\frac{x}{z}. We have 2a2+1ab+1a+1=2y2x2+xyzy+xy+1=2y3+x3x2(x+y+z)\frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1}=\frac{\frac{2 y^{2}}{x^{2}}+\frac{x}{y}}{\frac{z}{y}+\frac{x}{y}+1}=\frac{2 y^{3}+x^{3}}{x^{2}(x+y+z)}.

Hence cyc2a2+1ab+1a+1=1x+y+zcyc2y3+x3x2=1x+y+z(x+y+z+2cycy3x2)\sum_{cyc} \frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1}=\frac{1}{x+y+z} \cdot \sum_{cyc} \frac{2 y^{3}+x^{3}}{x^{2}}=\frac{1}{x+y+z} \cdot\left(x+y+z+2 \sum_{cyc} \frac{y^{3}}{x^{2}}\right).

By Rearrangement Inequality we get cycy3x2x+y+z\sum_{\text{cyc}} \frac{y^{3}}{x^{2}} \geq x+y+z.

So cyc2a2+1ab+1a+11x+y+z(3x+3y+3z)=3\sum_{\text{cyc}} \frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1} \geq \frac{1}{x+y+z} \cdot(3x+3y+3z)=3.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.