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Algebra Difficulty 7.5 National olympiad, round 2 Prove it

45. Let a,b,ca, b, c be positive real numbers, prove that
(2+ab)2+(2+bc)2+(2+ca)29(a+b+c)2ab+bc+ca\left(2+\frac{a}{b}\right)^{2}+\left(2+\frac{b}{c}\right)^{2}+\left(2+\frac{c}{a}\right)^{2} \geq \frac{9(a+b+c)^{2}}{a b+b c+c a} (Pham Kim Hung)

Solution

Prove that the inequality is equivalent to
cyca2b2+4cycab9(a2+b2+c2)2ab+bc+ca+6\sum_{c y c} \frac{a^{2}}{b^{2}}+4 \sum_{c y c} \frac{a}{b} \geq \frac{9\left(a^{2}+b^{2}+c^{2}\right)^{2}}{a b+b c+c a}+6

Consider the following identities:
ab+bc+ca3=(ab)2ab+(ca)(cb)aca2+b2+c2abbcca=(ab)2+(bc)2+(ca)2\begin{array}{l} \frac{a}{b}+\frac{b}{c}+\frac{c}{a}-3=\frac{(a-b)^{2}}{a b}+\frac{(c-a)(c-b)}{a c} \\ a^{2}+b^{2}+c^{2}-a b-b c-c a=(a-b)^{2}+(b-c)^{2}+(c-a)^{2} \end{array}

The inequality becomes the following form:
(ab)2M+(ca)(cb)N0(a-b)^{2} M+(c-a)(c-b) N \geq 0

where M=4ab+(a+b)2a2b29ab+bc+ca,N=4ac+(c+a)(c+b)a2c29ab+bc+ca M=\frac{4}{a b}+\frac{(a+b)^{2}}{a^{2} b^{2}}-\frac{9}{a b+b c+c a}, \quad N=\frac{4}{a c}+\frac{(c+a)(c+b)}{a^{2} c^{2}}-\frac{9}{a b+b c+c a}
Notice that, if abc a \geq b \geq c , then
cycabcycba;cyca2b2cycb2a2\sum_{c y c} \frac{a}{b} \leq \sum_{c y c} \frac{b}{a} ; \quad \sum_{c y c} \frac{a^{2}}{b^{2}} \leq \sum_{c y c} \frac{b^{2}}{a^{2}}

Therefore, we only need to consider the case abc a \geq b \geq c , because the case abc a \geq b \geq c after applying (*) will reduce its value
N5ac+bac29ab+bc+ca>0M+N8ab+5ac18ab+bc+ca>0\begin{array}{l} N \geq \frac{5}{a c}+\frac{b}{a c^{2}}-\frac{9}{a b+b c+c a}>0 \\ M+N \geq \frac{8}{a b}+\frac{5}{a c}-\frac{18}{a b+b c+c a}>0 \end{array}

Let k=1+52 k=\frac{1+\sqrt{5}}{2} , consider the following cases:
(i) The case abk(bc) a-b \leq k(b-c) . Then we have (ab)2(ac)(bc) (a-b)^{2} \leq(a-c)(b-c) , so
(ab)2M+(ca)(cb)N(ab)2(M+N)0(a-b)^{2} M+(c-a)(c-b) N \geq(a-b)^{2}(M+N) \geq 0
(ii) The case abk(bc) a-b \geq k(b-c) . We only need to prove M0 M \geq 0 or
(a2+b2+6ab)(ab+bc+ca)9a2b2\left(a^{2}+b^{2}+6 a b\right)(a b+b c+c a) \geq 9 a^{2} b^{2}

Since (k+1)bakc (k+1) b-a \leq k c and k+1=3+52 k+1=\frac{3+\sqrt{5}}{2} , we have
ab(ab)2=[(3+5)b2a][a(35)b2]kc[a+(35)b2]2c(a+b)a b-(a-b)^{2}=\left[\frac{(3+\sqrt{5}) b}{2}-a\right]\left[a-\frac{(3-\sqrt{5}) b}{2}\right] \leq k c\left[a+\frac{(3-\sqrt{5}) b}{2}\right] \leq 2 c(a+b)

Therefore
(a2+b2+6ab)(ab+bc+ca)9a2b2ab[(a2b2)2ab]+c(a+b)3>c(a+b)34abc(a+b)=c(a+b)(ab)20\begin{array}{l} \left(a^{2}+b^{2}+6 a b\right)(a b+b c+c a)-9 a^{2} b^{2} \geq a b\left[\left(a^{2}-b^{2}\right)^{2}-a b\right]+c(a+b)^{3} \\ >c(a+b)^{3}-4 a b c(a+b)=c(a+b)(a-b)^{2} \geq 0 \end{array}

The equality holds if and only if a=b=c a=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.