Inequality of Numerators

Algebra Level 2

Let a , b , c a,b,c be positive reals. Also, let k k be the largest possible real such that

a 1 + b 1 + c 1 + a + b 1 + b + c 1 + c + a 1 a + b + c k . \dfrac{a}{1}+\dfrac{b}{1}+\dfrac{c}{1}+\dfrac{a+b}{1}+\dfrac{b+c}{1}+\dfrac{c+a}{1}\le \dfrac{a+b+c}{k}.

If k k can be expressed as p q \frac{p}{q} for relatively prime positive integers p p and q q , then what is p + q ? p+q?



The answer is 4.

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5 solutions

Daniel Liu
Jul 18, 2014

We see that the LHS simply equals 3 ( a + b + c ) 3(a+b+c) . Dividing both sides by a + b + c a+b+c , we have 3 1 k 3\le \dfrac{1}{k}

Solving for k k : k 1 3 k\le \dfrac{1}{3} so our answer is 1 + 3 = 4 1+3=\boxed{4} .

Doing this while it is unrated is a real steal; you get 100 points, 90 more than what the problem is probably worth!

Daniel Liu - 6 years, 10 months ago

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Yes, but what use are points? :(

Sharky Kesa - 6 years, 10 months ago

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Showing your progress? I don't know. I wish you could buy the prizes now... :(

Daniel Liu - 6 years, 10 months ago

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@Daniel Liu Brilliant became too large to sustain these prizes. I remember the old days, when Cody Johnson had 300 followers and was the most popular member.

Sharky Kesa - 6 years, 10 months ago

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@Sharky Kesa I think its like Finn now.

Joshua Ong - 6 years, 9 months ago

I have a question... it's said in the question, p and q are prime positive integers. In the solution, 1 has been considered (1+3), but 1 is NOT a prime number. So what exactly is the answer?

Rajarshi Biswas - 6 years, 10 months ago

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They are relatively prime. That is different from prime. It means that they have no common factor.

Joshua Ong - 6 years, 9 months ago

@Daniel Liu , You also have to mention k 0 k\neq 0 (Just for perfection)

Karthik Sharma - 6 years, 10 months ago

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Well, that's pretty obvious, and the helpfulness of the statement is not enough to account for the extra amount of words needed to say it. So I won't put it on. Thanks for mentioning it though.

Daniel Liu - 6 years, 10 months ago

The solution is flawed; k 0 k\neq 0 is not the only mistake. The true conclusion had to be 0 < k 1 3 0<k\le \frac{1}{3} . k k can't be negative as well.

mathh mathh - 6 years, 10 months ago

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Can you explain? I don't see why k > 0 k > 0 is necessary. Also, how is my solution flawed?

Daniel Liu - 6 years, 10 months ago

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@Daniel Liu It is enough to know about the properties of hyperbolas to see that k < 0 k<0 never works. You can see this by taking, e.g., k = 1 k=-1 , which implies 3 1 1 = 1 3\le \frac{1}{-1}=-1 , which is false. And your solution, as I said, is flawed because you haven't stated all the restrictions on k k , which are 0 < k 1 3 0<k\le \frac{1}{3} .

mathh mathh - 6 years, 10 months ago

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@Mathh Mathh Why do I need 0 < k 1 3 0 < k \le \dfrac{1}{3} ? Is ther an alternate answer if I don't put that restriction? Can you just tell me where in my solution did I go wrong?

Daniel Liu - 6 years, 10 months ago

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@Daniel Liu Well, k 1 3 k\le \frac{1}{3} sure does mean that the answer is 1 3 \frac{1}{3} , but since Karthik Sharma was going about how k 0 k\neq 0 , I can go about how k > 0 k> 0 , but that's just nitpicking. Sorry if I exaggerated a bit.

mathh mathh - 6 years, 10 months ago

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@Mathh Mathh Okay, I see what you mean now. I actually didn't realize that the inequality would always be false if k k is negative.

Daniel Liu - 6 years, 10 months ago

Thank you..! :) #Daniel Liu

Siva Prasad - 6 years, 10 months ago

But 1 isn't a prime positive integer!

Andrea Forlani - 6 years, 10 months ago

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Relatives prime is not prime, it means they have GCD is 1

Nguyễn Hoàng - 6 months, 3 weeks ago

1 aint prime

Abhay Kanwar - 6 years, 9 months ago

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Relative prime, U need read twice the clues

Nguyễn Hoàng - 6 months, 3 weeks ago
Desrivina Ramkas
Aug 5, 2014

a 1 \frac{a}{1} + + b 1 \frac{b}{1} + + c 1 \frac{c}{1} + + a + b 1 \frac{a+b}{1} + + b + c 1 \frac{b+c}{1} + + c + a 1 \frac{c+a}{1} a + b + c k \frac{a+b+c}{k}

a + b + c + ( a + b ) + ( b + c ) + ( c + a ) 1 \frac{a+b+c+(a+b)+(b+c)+(c+a)}{1} a + b + c k \frac{a+b+c}{k}

3 a + 3 b + 3 c 1 \frac{3a+3b+3c}{1} a + b + c k \frac{a+b+c}{k}

3 ( a + b + c ) 3(a+b+c) a + b + c k \frac{a+b+c}{k}

3 k 3k a + b + c a + b + c \frac{a+b+c}{a+b+c}

3 k 3k 1 1

k k 1 3 \frac{1}{3}

so, p q \frac{p}{q} = 1 3 \frac{1}{3}

p p + + q q = 1 1 + + 3 3

p p + + q q = 4 4

Robert Ladog
May 4, 2020

3(a+b+c)<=(a+b+c)/k Let x = a+b+c 3x = x/k k= 1/3 p+q = 4

Siva Prasad
Jul 18, 2014

(a+b+c)/k must be equal to 3(a+b+c). so the value of k should be '1/3'. Then 1+3=4

Sai Venkat
Jul 31, 2014

let us assume a=b=c=1;then evaluating the question we get 4

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