Minimizing Inequality

Algebra Level 4

( 1 1 + 2 a b c 2 + 1 1 + 2 b c a 2 + 1 1 + 2 c a b 2 ) 2 \left(\dfrac{1}{1+2ab-c^2}+\dfrac{1}{1+2bc-a^2}+\dfrac{1}{1+2ca-b^2}\right)^2

Let a , b a,b and c c be non-negative real numbers such that a 2 + b 2 + c 2 = 1 a^2+b^2+c^2=1 .

The minimum value of the expression above can be expressed as p q \dfrac{p}{q} for positive coprime integers p , q p,q . Find p + q p+q .


The answer is 97.

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2 solutions

Dinesh Chavan
May 21, 2014

For this, we make use of a simplified version of Cauchy Schwartz inequality, which states; a 2 b + c 2 d + e 2 f ( a + c + e ) 2 b + d + f \frac{a^2}{b}+\frac{c^2}{d}+\frac{e^2}{f}≥\frac{(a+c+e)^2}{b+d+f} Therefore , moving to the question again, (Here, I will ignore the outer square this moment and use it later)

So, by applying Cauchy Schwartz inequality, we get, 1 1 + 2 a b c 2 + 1 1 + 2 b c a 2 + 1 1 + 2 c a b 2 ( 1 + 1 + 1 ) 2 1 + 1 + 1 ( a 2 + b 2 + c 2 ) + 2 ( a b + b c + c a ) \frac{1}{1+2ab-c^2}+\frac{1}{1+2bc-a^2}+\frac{1}{1+2ca-b^2}≥\frac{(1+1+1)^2}{1+1+1-(a^2+b^2+c^2)+2(ab+bc+ca)} Which gives ; 9 2 + 2 ( a b + b c + c a ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( 1 ) ≥\frac{9}{2+2(ab+bc+ca)}..............................(1)

Now by simple A M G M AM-GM inequality, we get, a 2 + b 2 + c 2 a b + b c + c a a^2+b^2+c^2≥ab+bc+ca

Now, in order to minimize ( 1 ) (1) , we must consider the equality case and should take the denominator to be maximum, So, a b + b c + c a ab+bc+ca is maximum at 1 1 , since a 2 + b 2 + c 2 = 1 a^2+b^2+c^2=1

Therefore, putting the above results in ( 1 ) (1) , we get the minimum as, M i n i m u m = 9 2 + 2 = 9 4 Minimum=\frac{9}{2+2}=\frac{9}{4}

Therefore, according to the question, we get, Minimum of

( 1 1 + 2 a b c 2 + 1 1 + 2 b c a 2 + 1 1 + 2 c a b 2 ) 2 = ( 9 4 ) 2 = 81 16 = p q (\frac{1}{1+2ab-c^2}+\frac{1}{1+2bc-a^2}+\frac{1}{1+2ca-b^2})^2=(\frac{9}{4})^2=\frac{81}{16}=\frac{p}{q} Therefore, p + q = 81 + 16 = 97 p+q=81+16=97

I too did the same (Titu's lemma)

Mehul Chaturvedi - 6 years, 4 months ago

Damn I forgot to square it and kept entering 13.........

Arjun Bharat - 7 years ago

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LOL, Expected mistake.

Dinesh Chavan - 7 years ago
Daniel Liu
May 21, 2014

We want to find the minimum value of ( c y c 1 1 + 2 a b c 2 ) 2 \left(\sum_{cyc}\dfrac{1}{1+2ab-c^2}\right)^2

First, homogenize by multiplying by ( a 2 + b 2 + c 2 ) 2 (a^2+b^2+c^2)^2 : ( a 2 + b 2 + c 2 ) 2 ( c y c 1 1 + 2 a b c 2 ) 2 (a^2+b^2+c^2)^2\left(\sum_{cyc}\dfrac{1}{1+2ab-c^2}\right)^2

Juggle some powers: ( ( a 2 + b 2 + c 2 ) c y c ( 1 1 + 2 a b c 2 ) ) 2 \left((a^2+b^2+c^2)\sum_{cyc}\left(\dfrac{1}{1+2ab-c^2}\right)\right)^2

We apply Cauchy on the inside to get ( ( a 2 + b 2 + c 2 ) c y c ( 1 1 + 2 a b c 2 ) ) 2 ( c y c a 1 + 2 b c a 2 ) 4 \left((a^2+b^2+c^2)\sum_{cyc}\left(\dfrac{1}{1+2ab-c^2}\right)\right)^2\ge \left(\sum_{cyc}\dfrac{a}{\sqrt{1+2bc-a^2}}\right)^4

Substituting 1 a 2 = b 2 + c 2 1-a^2=b^2+c^2 to each of the fractions: ( c y c a 1 + 2 b c a 2 ) 4 = ( c y c a b 2 + 2 b c + c 2 ) 4 \left(\sum_{cyc}\dfrac{a}{\sqrt{1+2bc-a^2}}\right)^4= \left(\sum_{cyc}\dfrac{a}{\sqrt{b^2+2bc+c^2}}\right)^4

Thankfully, this gets rid of the square roots: ( c y c a b 2 + 2 b c + c 2 ) 4 = ( c y c a b + c ) 4 \left(\sum_{cyc}\dfrac{a}{\sqrt{b^2+2bc+c^2}}\right)^4= \left(\sum_{cyc}\dfrac{a}{b+c}\right)^4

However, c y c a b + c 3 2 \sum_{cyc}\dfrac{a}{b+c}\ge \dfrac{3}{2} is true because that is the exact statement of Nesbitt's Inequality. Thus, ( c y c a b + c ) 4 ( 3 2 ) 4 = 81 16 \left(\sum_{cyc}\dfrac{a}{b+c}\right)^4\ge \left(\dfrac{3}{2}\right)^4=\dfrac{81}{16}

Thus, our answer is 81 + 16 = 97 81+16=\boxed{97}

Equality case is when a = b = c = 3 3 a=b=c=\dfrac{\sqrt{3}}{3} .

The idea of homogenize means to make each term in the expression have the same degree (not necessarily 0). So, as opposed to converting the numerator to a 2 + b 2 + c 2 a^2 + b^2 + c^2 , you should have replaced 1 in the denominator with a 2 + b 2 + c 2 a^2 + b^2 + c^2 .

Doing so makes the denominator 1 + 2 a b c 2 = a 2 + 2 a b + b 2 = ( a + b ) 2 1 + 2ab - c^2 = a^2 + 2ab + b^2 = (a+b)^2 , and it's much easier to continue from here.

Calvin Lin Staff - 7 years ago

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ohhh, I see. Thanks for clarifying.

Daniel Liu - 7 years ago

I proceeded to that step and got stuck....and though I saw a note relating to Cauchy Schwartz...I haven't actually got the hang of it....seeing the symmetric expression, I too the value of a=b=c=(1/root(3)) and substituted the values in the expression. Is there a way to do it with ought this guesswork or Cauchy?

Tanya Gupta - 6 years, 11 months ago

As far as I know, homogenization means to make each term in an expression to have the same degree. How come the manipulation you performed in step 2 is homogenization ?

Aditya Sky - 3 years, 5 months ago

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