Maths Olympiad Prep

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Algebra Difficulty 6.1 National olympiad Prove it

Example 5 Given that a,b,ca, b, c are positive real numbers. Prove:
aa2+8bc+bb2+8ac+cc2+8ab1 \frac{a}{\sqrt{a^{2}+8 b c}}+\frac{b}{\sqrt{b^{2}+8 a c}}+\frac{c}{\sqrt{c^{2}+8 a b}} \geqslant 1 \text {. }
(42nd IMO)

Solution

Prove: Introduce the parameter λ\lambda, set
aa2+8bcaλaλ+bλ+cλ, \frac{a}{\sqrt{a^{2}+8 b c}} \geqslant \frac{a^{\lambda}}{a^{\lambda}+b^{\lambda}+c^{\lambda}},

i.e., (aλ+bλ+cλ)2a2(aλ)2(a2+8bc)\left(a^{\lambda}+b^{\lambda}+c^{\lambda}\right)^{2} \cdot a^{2} \geqslant\left(a^{\lambda}\right)^{2}\left(a^{2}+8 b c\right).
And (aλ+bλ+cλ)2(aλ)2\left(a^{\lambda}+b^{\lambda}+c^{\lambda}\right)^{2}-\left(a^{\lambda}\right)^{2}
=(bλ+cλ)(aλ+bλ+cλ+aλ)=\left(b^{\lambda}+c^{\lambda}\right)\left(a^{\lambda}+b^{\lambda}+c^{\lambda}+a^{\lambda}\right)
2(bc)λ24aλ2(bc)λ4=8aλ2(bc)3λ4\geqslant 2(b c)^{\frac{\lambda}{2}} \cdot 4 a^{\frac{\lambda}{2}}(b c)^{\frac{\lambda}{4}}=8 a^{\frac{\lambda}{2}}(b c)^{\frac{3 \lambda}{4}},

then
(aλ+bλ+cλ)2(aλ)2+8aλ2(bc)3λ4\left(a^{\lambda}+b^{\lambda}+c^{\lambda}\right)^{2} \geqslant\left(a^{\lambda}\right)^{2}+8 a^{\frac{\lambda}{2}}(b c)^{\frac{3 \lambda}{4}} =aλ2[a3λ2+8(bc)3λ4]=a^{\frac{\lambda}{2}}\left[a^{\frac{3 \lambda}{2}}+8(b c)^{\frac{3 \lambda}{4}}\right].
Take λ=43\lambda=\frac{4}{3}, then we have
(a43+b43+c43)2a23(a2+8bc)\left(a^{\frac{4}{3}}+b^{\frac{4}{3}}+c^{\frac{4}{3}}\right)^{2} \geqslant a^{\frac{2}{3}}\left(a^{2}+8 b c\right),

i.e., \square
aa2+8bca43a43+b43+c43\frac{a}{\sqrt{a^{2}+8 b c}} \geqslant \frac{a^{\frac{4}{3}}}{a^{\frac{4}{3}}+b^{\frac{4}{3}}+c^{\frac{4}{3}}}.
Similarly, bb2+8acb43a43+b43+c43\frac{b}{\sqrt{b^{2}+8 a c}} \geqslant \frac{b^{\frac{4}{3}}}{a^{\frac{4}{3}}+b^{\frac{4}{3}}+c^{\frac{4}{3}}},
cc2+8abc43a43+b43+c43\frac{c}{\sqrt{c^{2}+8 a b}} \geqslant \frac{c^{\frac{4}{3}}}{a^{\frac{4}{3}}+b^{\frac{4}{3}}+c^{\frac{4}{3}}}.
Adding the above three inequalities yields the conclusion.
Note: The above three examples are inequalities with cyclic symmetry. The proof of such inequalities often involves constructing a class of fractions, and these fractions can be cyclically added to a constant. The parameter is introduced as a power index, and the inequality is solved in reverse to determine the parameter, thereby achieving the proof of the inequality. Introducing the parameter as a power index is an advanced technique, with higher requirements for transformation, and its practical application is not particularly widespread.

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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.