# Homework Help: Proof this inequality using Chebyshev's sum inequality

1. Mar 23, 2007

### playboy

1. The problem statement, all variables and given/known data

Let a,b,c,d,e be positive real numbers. Show that

$$\displaystyle{\frac{a}{b+c}} + \displaystyle{\frac{b}{c+d}} + \displaystyle{\frac{c}{d+e}} + \displaystyle{\frac{d}{e+a}} +\displaystyle{\frac{e}{a+b}} \geq \displaystyle{\frac{5}{2}}$$

2. Relevant equations

Chebyshev's sum inequality:
http://en.wikipedia.org/wiki/Chebyshev's_sum_inequality

3. The attempt at a solution

Assume: a > b > c > d > e

Then: a+b > a+e > b+c > c+d > d+e

Or: $$\displaystyle{\frac{1}{d+e}} \geq \displaystyle{\frac{1}{c+d}} \geq \displaystyle{\frac{1}{b+c}} \geq \displaystyle{\frac{1}{a+e}} \geq \displaystyle{\frac{1}{a+b}}$$

Hence, we have:

$a \geq b \geq c \geq d \geq e \geq$

$$\displaystyle{\frac{1}{d+e}} \geq \displaystyle{\frac{1}{c+d}} \geq \displaystyle{\frac{1}{b+c}} \geq \displaystyle{\frac{1}{a+e}} \geq \displaystyle{\frac{1}{a+b}}$$

But...this $$\sum a_{k}*b_{k}$$ is NOT matching up like it what the question is asking...

Am I arranging these numbers wrong?

What I am trying to say is:

$$\displaystyle{\frac{a}{d+e}} + \displaystyle{\frac{b}{c+d}} + \displaystyle{\frac{c}{b+c}} + \displaystyle{\frac{d}{a+e}} + \displaystyle{\frac{e}{a+b}}$$

IS NOT EQUAL TO

$$\displaystyle{\frac{a}{b+c}} + \displaystyle{\frac{b}{c+d}} + \displaystyle{\frac{c}{d+e}} + \displaystyle{\frac{d}{e+a}} +\displaystyle{\frac{e}{a+b}} \geq \displaystyle{\frac{5}{2}}$$

Last edited by a moderator: Mar 23, 2007
2. Mar 23, 2007

### AKG

First, you can't assume a > b > c > d > e, since the sum on the left is not invariant under arbitrary permutations (it is invariant under cyclic permutations (abcde)k though). Also, how are you getting a+e > b+c? That's not a valid inference. And yes, the thing you end up with doesn't match up anyways.