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Algebra Difficulty 6.3 National olympiad Prove it Iran

Let aa, bb, cc be positive real numbers such that ab+bc+ca=1ab + bc + ca = 1. Show that

3(a+b+c)aabc+bbca+ccab. \sqrt{3}(\sqrt{a} + \sqrt{b} + \sqrt{c}) \le \frac{a\sqrt{a}}{bc} + \frac{b\sqrt{b}}{ca} + \frac{c\sqrt{c}}{ab}.

Solution

Solution 1. By Holder Inequality we have
(cycaabc)(cycbc)(cyc1)(cyca)3cycaabc13(cyca)3. \left(\sum_{cyc} \frac{a\sqrt{a}}{bc}\right)\left(\sum_{cyc} bc\right)\left(\sum_{cyc} 1\right) \geq \left(\sum_{cyc} \sqrt{a}\right)^3 \Rightarrow \sum_{cyc} \frac{a\sqrt{a}}{bc} \geq \frac{1}{3}\left(\sum_{cyc} \sqrt{a}\right)^3.
So it suffices to have
13(cyca)33(cyca)(cyca)233cyca274 \frac{1}{3}\left(\sum_{cyc} \sqrt{a}\right)^3 \geq \sqrt{3}\left(\sum_{cyc} \sqrt{a}\right) \Leftrightarrow \left(\sum_{cyc} \sqrt{a}\right)^2 \geq 3\sqrt{3} \Leftrightarrow \sum_{cyc} \sqrt{a} \geq \sqrt[4]{27}
Now suppose that cyca<274\sum_{cyc} \sqrt{a} < \sqrt[4]{27}. It suffices to prove that cycaabc3274\sum_{cyc} \frac{a\sqrt{a}}{bc} \geq \sqrt{3\sqrt[4]{27}}, but
we have cycaabc3cycaabc3=3(abc)16\sum_{cyc} \frac{a\sqrt{a}}{bc} \geq 3\sqrt[3]{\prod_{cyc} \frac{a\sqrt{a}}{bc}} = 3(abc)^{-\frac{1}{6}} (*) by AM-GM inequality
1=cycab3(abc)23(abc)233(abc)1634cycaabc3(abc)16334=354=312334=3274. \begin{align*} 1 &= \sum_{cyc} ab \geq 3(abc)^{\frac{2}{3}} \Rightarrow (abc)^{-\frac{2}{3}} \geq 3 \Rightarrow (abc)^{-\frac{1}{6}} \geq \sqrt[4]{3} \\ \Rightarrow \sum_{cyc} \frac{a\sqrt{a}}{bc} &\geq 3(abc)^{-\frac{1}{6}} \geq 3\sqrt[4]{3} = 3^{\frac{5}{4}} = 3^{\frac{1}{2}} \cdot 3^{\frac{3}{4}} = \sqrt{3\sqrt[4]{27}}. \end{align*}


Solution 2. First by Cauchy-Schwarz inequality we have
(cyca2bca)(cycbca)(cyca)2. \left(\sum_{cyc} \frac{a^2}{bc\sqrt{a}}\right)\left(\sum_{cyc} bc\sqrt{a}\right) \geq \left(\sum_{cyc} a\right)^2.
So it suffices to prove that
(cyca)2cycab3×abc(cycbc)(cyca). \left(\sum_{cyc} a\right)^2 \sqrt{\sum_{cyc} ab} \geq \sqrt{3} \times \sqrt{abc} \left(\sum_{cyc} \sqrt{bc}\right)\left(\sum_{cyc} \sqrt{a}\right).
Since cycab=1\sum_{cyc} ab = 1.
But by AM-GM and Cauchy-Schwarz we have
cycab13cycab(Cauchy-Schwarz)(1) \sqrt{\sum_{cyc} ab} \geq \frac{1}{\sqrt{3}} \sum_{cyc} \sqrt{ab} \quad (\text{Cauchy-Schwarz}) \quad (1)
cyca13(cyca)2(Cauchy-Schwarz)(2) \sum_{cyc} a \geq \frac{1}{3} \left( \sum_{cyc} \sqrt{a} \right)^2 \quad (\text{Cauchy-Schwarz}) \quad (2)

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