Let a , b , c be positive reals. Also, let 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 .
If k can be expressed as q p for relatively prime positive integers p and q , then what is p + q ?
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Doing this while it is unrated is a real steal; you get 100 points, 90 more than what the problem is probably worth!
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Yes, but what use are points? :(
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Showing your progress? I don't know. I wish you could buy the prizes now... :(
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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.
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?
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They are relatively prime. That is different from prime. It means that they have no common factor.
@Daniel Liu , You also have to mention k = 0 (Just for perfection)
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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.
The solution is flawed; k = 0 is not the only mistake. The true conclusion had to be 0 < k ≤ 3 1 . k can't be negative as well.
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Can you explain? I don't see why k > 0 is necessary. Also, how is my solution flawed?
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@Daniel Liu – It is enough to know about the properties of hyperbolas to see that k < 0 never works. You can see this by taking, e.g., k = − 1 , which implies 3 ≤ − 1 1 = − 1 , which is false. And your solution, as I said, is flawed because you haven't stated all the restrictions on k , which are 0 < k ≤ 3 1 .
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@Mathh Mathh – Why do I need 0 < k ≤ 3 1 ? 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?
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@Daniel Liu – Well, k ≤ 3 1 sure does mean that the answer is 3 1 , but since Karthik Sharma was going about how k = 0 , I can go about how k > 0 , but that's just nitpicking. Sorry if I exaggerated a bit.
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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 is negative.
Thank you..! :) #Daniel Liu
But 1 isn't a prime positive integer!
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Relatives prime is not prime, it means they have GCD is 1
1 aint prime
1 a + 1 b + 1 c + 1 a + b + 1 b + c + 1 c + a ≤ k a + b + c
1 a + b + c + ( a + b ) + ( b + c ) + ( c + a ) ≤ k a + b + c
1 3 a + 3 b + 3 c ≤ k a + b + c
3 ( a + b + c ) ≤ k a + b + c
3 k ≤ a + b + c a + b + c
3 k ≤ 1
k ≤ 3 1
so, q p = 3 1
p + q = 1 + 3
p + q = 4
3(a+b+c)<=(a+b+c)/k Let x = a+b+c 3x = x/k k= 1/3 p+q = 4
(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
let us assume a=b=c=1;then evaluating the question we get 4
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We see that the LHS simply equals 3 ( a + b + c ) . Dividing both sides by a + b + c , we have 3 ≤ k 1
Solving for k : k ≤ 3 1 so our answer is 1 + 3 = 4 .