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Algebra Level 5

[ ( a + b ) 2 + ( b + c ) 2 + ( c + a ) 2 ] [ 1 ( a b ) 2 + 1 ( b c ) 2 + 1 ( c a ) 2 ] [(a+b)^2+(b+c)^2+(c+a)^2] \left[\frac{1}{(a-b)^2}+\frac{1}{(b-c)^2}+\frac{1}{(c-a)^2}\right]

Given that a , b a,b and c c are distinct non-negative real numbers, and let the minimum value of the expression above be denoted as Q Q , find 1000 Q \lfloor 1000Q \rfloor .


Inspiration .


The answer is 30547.

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1 solution

Son Nguyen
Feb 9, 2016

Without losing the generally,we may assume:c=min{a,b,c}and 0 c < a , b \rightarrow 0\leq c< a,b We have: Q [ ( a + b ) 2 + a 2 + b 2 ] [ 1 ( a b ) 2 + 1 a 2 + 1 b 2 ] = 2 a 2 + a b + b 2 ( a b ) 2 + 2 ( a b + b a ) 2 + 2 ( a b + b a ) Q\geq[(a+b)^2+a^2+b^2][\frac{1}{(a-b)^2}+\frac{1}{a^2}+\frac{1}{b^2}]=2\frac{a^2+ab+b^2}{(a-b)^2}+2(\frac{a}{b}+\frac{b}{a})^2+2(\frac{a}{b}+\frac{b}{a}) Let t = a b + b a t=\frac{a}{b}+\frac{b}{a} , t > 2 t> 2 so Q f ( t ) = 2 ( t 2 + t + t + 1 t 2 ) Q\geq f(t)=2(t^2+t+\frac{t+1}{t-2}) Let considering function f ( t ) = 2 ( t 2 + t + t + 1 t 2 ) f(t)=2(t^2+t+\frac{t+1}{t-2}) in the range ( 2 ; + ) (2;+\infty ) we have: f ( t ) = 2 [ 2 t + 1 3 ( t 2 ) 2 ] f'(t)=2[2t+1-\frac{3}{(t-2)^2}] f ( t ) = 0 ( 2 t + 1 ) ( t 2 ) 3 3 = 0 f'(t)=0\Leftrightarrow (2t+1)(t-2)^3-3=0 2 t 3 7 t 2 + 4 t + 1 = 0 t = 5 + 33 4 ( t > 2 ) \Leftrightarrow 2t^3-7t^2+4t+1=0\Leftrightarrow t=\frac{5+\sqrt{33}}{4}(t> 2) Because of t = 5 + 33 4 t=\frac{5+\sqrt{33}}{4} so f'(t) reversal from negative to positive so f ( t ) m i n f(t)_{min} only when t = 5 + 33 4 t=\frac{5+\sqrt{33}}{4} So f ( t ) f ( 5 + 33 4 ) = 59 + 11 3 4 f(t)\geq f(\frac{5+\sqrt{33}}{4})=\frac{59+11\sqrt{3}}{4} f ( t ) m i n = 59 + 11 33 4 f(t)_{min}=\frac{59+11\sqrt{33}}{4} .The equality holds when a b + b a = 5 + 33 4 , c = 0 \frac{a}{b}+\frac{b}{a}=\frac{5+\sqrt{33}}{4},c=0 .Note that a,b,c maybe permutation. So Q = 59 + 11 33 4 = 30.547547278 Q=\frac{59+11\sqrt{33}}{4}=30.547547278 ;and 1000 Q = 30547.547278 = 30547 \left \lfloor 1000Q \right \rfloor=\left \lfloor 30547.547278 \right \rfloor=30547

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