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Algebra Difficulty 6.9 National olympiad Prove it

56. Let a,b,ca, b, c be positive real numbers, prove: ab+bc+ca3a2+b2+c2ab+bc+ca\frac{a}{b}+\frac{b}{c}+\frac{c}{a} \geq 3 \sqrt{\frac{a^{2}+b^{2}+c^{2}}{a b+b c+c a}} (Vo Quoc Ba Can)

Solution

Proof: Note that, if abca \geq b \geq c, then
(ab+bc+ca)(ba+cb+ac)=(ab)(ac)(cb)abc0\left(\frac{a}{b}+\frac{b}{c}+\frac{c}{a}\right)-\left(\frac{b}{a}+\frac{c}{b}+\frac{a}{c}\right)=\frac{(a-b)(a-c)(c-b)}{a b c} \leq 0

Therefore, we only need to consider the case abca \geq b \geq c. Squaring both sides, we have
cyca2b2+cyc2ba9(a2+b2+c2)ab+bc+ca\sum_{c y c} \frac{a^{2}}{b^{2}}+\sum_{c y c} \frac{2 b}{a} \geq \frac{9\left(a^{2}+b^{2}+c^{2}\right)}{a b+b c+c a}

Additionally, using the following identities
ba+cb+ac3=(bc)2bc+(ab)(ac)aca2b2+b2c2+c2a23=(bc)2(b+c)b2c2+(a2b2)(a2c2)a2b2a2+b2+c2(ab+bc+ca)=(bc)2+(ab)(ac)\begin{array}{l} \frac{b}{a}+\frac{c}{b}+\frac{a}{c}-3=\frac{(b-c)^{2}}{b c}+\frac{(a-b)(a-c)}{a c} \\ \frac{a^{2}}{b^{2}}+\frac{b^{2}}{c^{2}}+\frac{c^{2}}{a^{2}}-3=\frac{(b-c)^{2}(b+c)}{b^{2} c^{2}}+\frac{\left(a^{2}-b^{2}\right)\left(a^{2}-c^{2}\right)}{a^{2} b^{2}} \\ a^{2}+b^{2}+c^{2}-(a b+b c+c a)=(b-c)^{2}+(a-b)(a-c) \end{array}

The inequality is equivalent to
(bc)2M+(ab)(ac)N0(b-c)^{2} M+(a-b)(a-c) N \geq 0

where M=2bc+(b+c)2b2c29ab+bc+caM=\frac{2}{b c}+\frac{(b+c)^{2}}{b^{2} c^{2}}-\frac{9}{a b+b c+c a};
N=2ac+(a+b)(a+c)a2b29ab+bc+caN=\frac{2}{a c}+\frac{(a+b)(a+c)}{a^{2} b^{2}}-\frac{9}{a b+b c+c a}

If bcabb-c \geq a-b, then 2(bc)2(ab)(ac)2(b-c)^{2} \geq(a-b)(a-c). Thus, we have
M6bc9ab+bc+ca0M+2N6bc18ab+bc+ca0\begin{array}{l} M \geq \frac{6}{b c}-\frac{9}{a b+b c+c a} \geq 0 \\ M+2 N \geq \frac{6}{b c}-\frac{18}{a b+b c+c a} \geq 0 \end{array}

Therefore, we have
(bc)2M+(ab)(ac)N12(ab)(ac)(M+2N)0 . (b-c)^{2} M+(a-b)(a-c) N \geq \frac{1}{2}(a-b)(a-c)(M+2 N) \geq 0 \text { . }

Otherwise, if bcabb-c \leq a-b, then 2ba+c2 b \leq a+c, of course, M0M \geq 0 and
N2ac+a+b+cab22ac+3ab(2+3)2ac+ab>9ab+bc+caN \geq \frac{2}{a c}+\frac{a+b+c}{a b^{2}} \geq \frac{2}{a c}+\frac{3}{a b} \geq \frac{(\sqrt{2}+\sqrt{3})^{2}}{a c+a b}>\frac{9}{a b+b c+c a}

Equality holds if and only if a=b=ca=b=c.

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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.