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Algebra Difficulty 5.6 AIME, harder Prove it JBMO

Problem:
Let aa, bb, cc be positive real numbers. Prove that
((3a2+1)2+2(1+3b)2)((3b2+1)2+2(1+3c)2)((3c2+1)2+2(1+3a)2)483 \left(\left(3 a^{2}+1\right)^{2}+2\left(1+\frac{3}{b}\right)^{2}\right)\left(\left(3 b^{2}+1\right)^{2}+2\left(1+\frac{3}{c}\right)^{2}\right)\left(\left(3 c^{2}+1\right)^{2}+2\left(1+\frac{3}{a}\right)^{2}\right) \geq 48^{3}
When does equality hold?

Solution

Solution:
Let xx be a positive real number. By AM-GM we have 1+x+x+x4x34\frac{1+x+x+x}{4} \geq x^{\frac{3}{4}}, or equivalently 1+3x4x341+3 x \geq 4 x^{\frac{3}{4}}. Using this inequality we obtain:
(3a2+1)216a3 and 2(1+3b)232b32 \left(3 a^{2}+1\right)^{2} \geq 16 a^{3} \text{ and } 2\left(1+\frac{3}{b}\right)^{2} \geq 32 b^{-\frac{3}{2}}
Moreover, by inequality of arithmetic and geometric means we have
f(a,b)=(3a2+1)2+2(1+3b)216a3+32b32=16(a3+b32+b32)48ab f(a, b)=\left(3 a^{2}+1\right)^{2}+2\left(1+\frac{3}{b}\right)^{2} \geq 16 a^{3}+32 b^{-\frac{3}{2}}=16\left(a^{3}+b^{-\frac{3}{2}}+b^{-\frac{3}{2}}\right) \geq 48 \frac{a}{b}
Therefore, we obtain
f(a,b)f(b,c)f(c,a)48ab48bc48ca=483 f(a, b) f(b, c) f(c, a) \geq 48 \cdot \frac{a}{b} \cdot 48 \cdot \frac{b}{c} \cdot 48 \cdot \frac{c}{a}=48^{3}
Equality holds only when a=b=c=1a=b=c=1.

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