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Post Posted: Sep 13, 2009, 2:42 pm • # 1 


If a,b,c are three positive real numbers, prove that \frac {a^{2}+1}{b+c}+\frac {b^{2}+1}{c+a}+\frac {c^{2}+1}{a+b}\ge 3
 
 
Post Posted: Sep 13, 2009, 2:47 pm • # 2 


makar wrote:
If a,b,c are three positive real numbers, prove that \frac {a^{2} + 1}{b + c} + \frac {b^{2} + 1}{c + a} + \frac {c^{2} + 1}{a + b}\ge 3

\sum \frac{a^2+1}{b+c} \ge \frac{2a}{b+c} \ge 3 (nebsit inequality)

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Post Posted: Sep 13, 2009, 3:27 pm • # 3 


it's equivalent to

\frac{(a-1)^2}{b+c}+\frac{(b-1)^2}{a+c}+\frac{(c-1)^2}{a+b}+\frac{2(a-b)^2}{(a+c)(b+c)}+\frac{2(b-c)^2}{(a+c)(a+b)}+\frac{2(a...

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Post Posted: Sep 13, 2009, 3:48 pm • # 4 


makar wrote:
If a,b,c are three positive real numbers, prove that \frac {a^{2} + 1}{b + c} + \frac {b^{2} + 1}{c + a} + \frac {c^{2} + 1}{a + b}\ge 3

The inequality is obviously true because (a^2 + 1)(b^2 + 1)(c^2 + 1) \geq (a + b)(b + c)(a + c) holds for all positive real numbers a,b and c.
 
 
Post Posted: Sep 15, 2009, 7:00 am • # 5 


rmo 2006 guys were too lucky!

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Post Posted: Dec 31, 2009, 2:16 am • # 6 


This is very easy problem.


Attachments:
proof.doc [16 KiB]
Downloaded 54 times
 
 
Post Posted: Dec 31, 2009, 6:34 am • # 7 


That is easily solved by AM-GM and cs
∑(a^2+1)/(b+c)≥∑2a/(b+c)≥∑2a^2/(ab+ac)≥2(∑a)^2/2∑bc≥3
 
 
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