Let a , b , c be positive reals. Let 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
If k can be expressed as q p for relatively prime positive integers p , q , then find p + q .
Inequality of Numerators , the easier version of this inequality.
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I bet some people tried to use Cauchy Schwarz, and ended up with k = 1 2 . Cauchy Schwarz, in this case, isn't strong enough!
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Started with Cauchy schwarz, ended up wrong 2 times, and then hit upon more elegent method, Nice trap @Daniel Liu
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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 which only has the solution ( a , b , c ) = ( 0 , 0 , 0 ) which is not allowed.
I did so at first.
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?
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( a + b + c ) ( ∑ a 1 + ∑ a + b 1 ) = ∑ a a + b + c + ∑ a + b a + b + c = ∑ a a + a b + c + ∑ a + b a + b + a + b c = 3 + ∑ a b + c + 3 + ∑ a + b c = 6 + ∑ a b + c + ∑ a + b c
better?
Where do the summations come from, and what are the summations of 1-infinity?
Very good question. I used Titu's lemma and ended up with 12.
yeah this was my first approach of solution
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.
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That's a common way to cheat inequalities: to just plug in a = b = c .
However, you didn't prove that it was the answer.
By AM-HM a 1 + b 1 + c 1 3 ≤ 3 a + b + c and a + b 1 + b + c 1 + c + a 1 3 ≤ 3 2 ( a + b + c ) .
So: ( a + b + c ) ( a 1 + b 1 + c 1 ) ≥ 9 and ( a + b + c ) ( a + b 1 + b + c 1 + a + c 1 ) ≥ 2 9 .
Adding gives k = 2 2 7 .
Equality is obtained iff a = b = c
Or...the sets {a,b,c} and { a 1 , b 1 , c 1 } are oppositely ordered so by rearrangement inequality:
3 = a a + b b + c c ≤ a b + b c + c a 3 = a a + b b + c c ≤ a c + b a + c c 3 = a a + b b + c c ≤ a a + b b + c c
Adding the three inequalities gives:
9 ≤ ( a + b + c ) ( a 1 + b 1 + c 1 )
Appying this inequality to the triple ( a + b , a + c , b + c ) gives:
9 ≤ 2 ( a + b + c ) ( a + b 1 + b + c 1 + a + c 1 )
Add and we get k = 2 2 7
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You could have just used Cauchy to directly get 9 ≤ ( a + b + c ) ( a 1 + b 1 + c 1 )
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.
Here's my solution:
We shall normalize by assuming a + b + c = 1 . Then, the inequality reduces to c y c ∑ a 1 + c y c ∑ 1 − a 1 ≥ k now, let f ( x ) = x 1 + 1 − x 1 . Since f ( x ) a convex function, by Jensen's Inequality, we have f ( 3 a + b + c ) ≤ 3 f ( a ) + f ( b ) + f ( c ) \] so, f ( a ) + f ( b ) + f ( c ) ≥ 2 2 7 . But we also know that ∑ c y c a 1 + ∑ c y c 1 − a 1 = f ( a ) + f ( b ) + f ( c ) . And hence we have k = 2 2 7 , so p + q = 2 7 + 2 = 2 9
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Multiply a + b + c on both sides and simplify to get 6 + ∑ c a + b + ∑ b + c a ≥ k
We see that ∑ c a + b = ∑ c a + c b ≥ 6 6 b a ⋅ a b ⋅ c a ⋅ a c ⋅ b c ⋅ c b = 6 by six-variable AM-GM.
Also, ∑ b + c a ≥ 2 3 by Nesbitt's Inequality.
So 6 + 6 + 2 3 ≥ k ⟹ k ≤ 2 2 7 and our answer is 2 7 + 2 = 2 9 .