Generalized Inequality!

Algebra Level pending

Let a 1 , a 2 , . . . , a n a_{1}, a_{2},...,a_{n} be positive reals such that c y c a 1 a 2 + a 3 m \sum_{cyc} \frac{a_{1}}{a_{2}+a_{3}} \geq m where m m is maximum. Find the value of n m \frac{n}{m}


The answer is 2.

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1 solution

Mohammed Imran
Apr 4, 2020

By Rearrangement Inequality, we have c y c a 1 a 2 + a 3 a 1 a 1 + a n \sum_{cyc} \frac{a_{1}}{a_{2}+a_{3}} \geq \frac{a_{1}}{a_{1}+a_{n}} and c y c a 1 a 2 + a 3 a n a 1 + a n \sum_{cyc} \frac{a_{1}}{a_{2}+a_{3}} \geq \frac{a_{n}}{a_{1}+a_{n}} adding them up, we have c y c a 1 a 2 + a 3 n 2 \sum_{cyc} \frac{a_{1}}{a_{2}+a_{3}} \geq \frac{n}{2} and hence, we have m = n 2 m=\frac{n}{2} , so m n = 2 \frac{m}{n}=\boxed{2}

Nice solution Mohammed Imran. I just assumed that equality occurs so all the terms are 1/2 which gives m=n/2 and hence, n/m=2. So I recommend that the question should be framed in such a way that the answer can't be derived by "fluke methods"

Nitin Kumar - 1 year, 1 month ago

Oh. Thank you Nitin!

Mohammed Imran - 1 year, 1 month ago

Hi Imran. Do you mind sending a more detailed solution on where Rearrangement inequality was actually used? Thanks!

Nitin Kumar - 1 year, 1 month ago

ok. Thank you!

Mohammed Imran - 1 year, 1 month ago

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