Example 5 Given that a,b,c are positive real numbers. Prove: a2+8bca+b2+8acb+c2+8abc⩾1. (42nd IMO)
Solution
Prove: Introduce the parameter λ, set a2+8bca⩾aλ+bλ+cλaλ,
i.e., (aλ+bλ+cλ)2⋅a2⩾(aλ)2(a2+8bc). And (aλ+bλ+cλ)2−(aλ)2 =(bλ+cλ)(aλ+bλ+cλ+aλ) ⩾2(bc)2λ⋅4a2λ(bc)4λ=8a2λ(bc)43λ,
then (aλ+bλ+cλ)2⩾(aλ)2+8a2λ(bc)43λ=a2λ[a23λ+8(bc)43λ]. Take λ=34, then we have (a34+b34+c34)2⩾a32(a2+8bc),
i.e., □ a2+8bca⩾a34+b34+c34a34. Similarly, b2+8acb⩾a34+b34+c34b34, c2+8abc⩾a34+b34+c34c34. 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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