Inequality of Denominators

Algebra Level 5

Let a , b , c a,b,c be positive reals. Let k k be the largest possible real such that 1 a + 1 b + 1 c + 1 a + b + 1 b + c + 1 c + a k a + b + c \dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}+\dfrac{1}{a+b}+\dfrac{1}{b+c}+\dfrac{1}{c+a}\ge \dfrac{k}{a+b+c}

If k k can be expressed as p q \dfrac{p}{q} for relatively prime positive integers p , q p,q , then find p + q p+q .


Inequality of Numerators , the easier version of this inequality.


The answer is 29.

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5 solutions

Daniel Liu
Jul 18, 2014

Multiply a + b + c a+b+c on both sides and simplify to get 6 + a + b c + a b + c k 6+\sum \dfrac{a+b}{c}+\sum\dfrac{a}{b+c}\ge k

We see that a + b c = a c + b c 6 a b b a a c c a c b b c 6 = 6 \sum \dfrac{a+b}{c} =\sum \dfrac{a}{c}+\dfrac{b}{c} \ge 6\sqrt[6]{\dfrac{a}{b}\cdot \dfrac{b}{a}\cdot\dfrac{a}{c}\cdot \dfrac{c}{a}\cdot\dfrac{c}{b}\cdot \dfrac{b}{c} }=6 by six-variable AM-GM.

Also, a b + c 3 2 \sum\dfrac{a}{b+c}\ge \dfrac{3}{2} by Nesbitt's Inequality.

So 6 + 6 + 3 2 k k 27 2 6+6+\dfrac{3}{2} \ge k\implies k\le \dfrac{27}{2} and our answer is 27 + 2 = 29 27+2=\boxed{29} .

I bet some people tried to use Cauchy Schwarz, and ended up with k = 12 k=12 . Cauchy Schwarz, in this case, isn't strong enough!

Daniel Liu - 6 years, 10 months ago

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Started with Cauchy schwarz, ended up wrong 2 times, and then hit upon more elegent method, Nice trap @Daniel Liu

Dinesh Chavan - 6 years, 10 months ago

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It wasn't intended to be a trap, but it just happened to exist :P

An easy way to check if your inequality is strong enough is to look at the equality case. Equality case of Cauchy is a 2 = b 2 = c 2 = ( a + b ) 2 = ( b + c ) 2 = ( c + a ) 2 a^2=b^2=c^2=(a+b)^2=(b+c)^2=(c+a)^2 which only has the solution ( a , b , c ) = ( 0 , 0 , 0 ) (a,b,c)=(0,0,0) which is not allowed.

Daniel Liu - 6 years, 10 months ago

I did so at first.

Fahim Shahriar Shakkhor - 6 years, 10 months ago

I can't catch the equation after you multiply it with a+b+c than you simplify it. Could you write more detail about how you simplify it?

Hafizh Ahsan Permana - 6 years, 10 months ago

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( a + b + c ) ( 1 a + 1 a + b ) = a + b + c a + a + b + c a + b = a a + b + c a + a + b a + b + c a + b = 3 + b + c a + 3 + c a + b = 6 + b + c a + c a + b \begin{aligned}(a+b+c)\left(\sum \dfrac{1}{a}+\sum \dfrac{1}{a+b}\right)&= \sum \dfrac{a+b+c}{a}+\sum \dfrac{a+b+c}{a+b}\\ &= \sum \dfrac{a}{a}+\dfrac{b+c}{a}+\sum \dfrac{a+b}{a+b}+\dfrac{c}{a+b}\\ &= 3+\sum \dfrac{b+c}{a}+3+\sum \dfrac{c}{a+b}\\ &= 6+\sum \dfrac{b+c}{a}+\sum \dfrac{c}{a+b}\end{aligned}

better?

Daniel Liu - 6 years, 10 months ago

Where do the summations come from, and what are the summations of 1-infinity?

Trevor Arashiro - 6 years, 10 months ago

Very good question. I used Titu's lemma and ended up with 12.

Kunal Verma - 6 years, 4 months ago

yeah this was my first approach of solution

Swayam Prakash Kar - 8 months, 3 weeks ago

Put a=b=c=1 and get the damn answer. A wild guess that the equality would exist if all of them were equal. I know that that is no method, but we Indians somehow find a way.

Sagar Torres - 6 years, 10 months ago

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That's a common way to cheat inequalities: to just plug in a = b = c a=b=c .

However, you didn't prove that it was the answer.

Daniel Liu - 6 years, 10 months ago
David Vaccaro
Jul 21, 2014

By AM-HM 3 1 a + 1 b + 1 c a + b + c 3 \frac{3}{\frac{1}{a}+\frac{1}{b}+\frac{1}{c}} \leq \frac{a+b+c}{3} and 3 1 a + b + 1 b + c + 1 c + a 2 ( a + b + c ) 3 \frac{3}{\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}} \leq \frac{2(a+b+c)}{3} .

So: ( a + b + c ) ( 1 a + 1 b + 1 c ) 9 (a+b+c)(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})\geq 9 and ( a + b + c ) ( 1 a + b + 1 b + c + 1 a + c ) 9 2 (a+b+c)(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{a+c})\geq \frac{9}{2} .

Adding gives k = 27 2 k=\frac{27}{2} .

Equality is obtained iff a = b = c a=b=c

Or...the sets {a,b,c} and { 1 a , 1 b , 1 c \frac{1}{a},\frac{1}{b}, \frac{1}{c} } are oppositely ordered so by rearrangement inequality:

3 = a a + b b + c c b a + c b + a c 3=\frac{a}{a}+\frac{b}{b}+ \frac{c}{c} \leq \frac{b}{a}+\frac{c}{b}+\frac{a}{c} 3 = a a + b b + c c c a + a b + c c 3=\frac{a}{a}+\frac{b}{b}+\frac{c}{c} \leq \frac{c}{a}+\frac{a}{b}+\frac{c}{c} 3 = a a + b b + c c a a + b b + c c 3=\frac{a}{a}+\frac{b}{b}+\frac{c}{c} \leq \frac{a}{a}+\frac{b}{b}+\frac{c}{c}

Adding the three inequalities gives:

9 ( a + b + c ) ( 1 a + 1 b + 1 c ) 9 \leq (a+b+c) (\frac{1}{a}+\frac{1}{b}+\frac{1}{c})

Appying this inequality to the triple ( a + b , a + c , b + c a+b,a+c,b+c ) gives:

9 2 ( a + b + c ) ( 1 a + b + 1 b + c + 1 a + c ) 9 \leq 2(a+b+c) (\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{a+c})

Add and we get k = 27 2 k=\frac{27}{2}

David Vaccaro - 6 years, 10 months ago

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You could have just used Cauchy to directly get 9 ( a + b + c ) ( 1 a + 1 b + 1 c ) 9\le (a+b+c)\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)

Daniel Liu - 6 years, 10 months ago

Ok,the solutions here are so creative. Maybe mine one is very poor but if you substitute a=b=c=1 since the value of k is independent of it. After simplifying , we get k is less than or equal to 27/2 where p= 27, q= 2 which adds upto 29. LOL.Not creative but time saving.

Mohammed Imran
Apr 4, 2020

Here's my solution:

We shall normalize by assuming a + b + c = 1 a+b+c=1 . Then, the inequality reduces to c y c 1 a + c y c 1 1 a k \sum_{cyc} \frac{1}{a}+\sum_{cyc} \frac{1}{1-a} \geq k now, let f ( x ) = 1 x + 1 1 x f(x)=\frac{1}{x}+\frac{1}{1-x} . Since f ( x ) f(x) a convex function, by Jensen's Inequality, we have f ( a + b + c 3 ) f ( a ) + f ( b ) + f ( c ) 3 f(\frac{a+b+c}{3}) \leq \frac{f(a)+f(b)+f(c)}{3} \] so, f ( a ) + f ( b ) + f ( c ) 27 2 f(a)+f(b)+f(c) \geq \frac{27}{2} . But we also know that c y c 1 a + c y c 1 1 a = f ( a ) + f ( b ) + f ( c ) \sum_{cyc} \frac{1}{a}+\sum_{cyc} \frac{1}{1-a}=f(a)+f(b)+f(c) . And hence we have k = 27 2 k=\frac{27}{2} , so p + q = 27 + 2 = 29 p+q=27+2=\boxed{29}

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