Proving Inequalities Involving Positive Real Numbers

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Homework Help Overview

The discussion revolves around proving an inequality involving positive real numbers \(a\), \(b\), and \(c\). The specific inequality to be proven is \(a^3 + b^3 + c^3 \geq a^2b + b^2c + c^2a\).

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore various approaches to factorizing the inequality and question the ease of such factorization. Some suggest that the problem may relate to known inequalities, while others propose manipulating the inequality by dividing through by \(abc\) and examining conditions under which the inequality holds.

Discussion Status

The discussion is ongoing, with participants sharing different insights and approaches. Some have offered potential methods for tackling the problem, while others express uncertainty about the factorization process. There is no explicit consensus on a single method or solution yet.

Contextual Notes

Participants are considering assumptions about the values of \(a\), \(b\), and \(c\), including the possibility of them being greater than 1, and discussing the implications of these assumptions on the inequality.

Sam Morse
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Homework Statement



Let a,b,c be positive real numbers. Prove that:

a3+b3+c3≥a2b+b2c+c2a

Homework Equations





The Attempt at a Solution



I assumed that a≥b≥c>0 following which I shifted the left side of this inequality to the right side giving

a3+b3+c3-(a2b+b2c+c2a)≥0

How do I do the required factorisation ... ?
 
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Are you sure that there is an easy way to factorize this?

I would expect that this can be reduced to a problem similar to a/b + b/c + c/a > 3.
 
I see a way to solve it, though it is a bit messy. First divide the entire equation by abc .
A bit of algebra gets a^3+b^3+c^3 \geq ab + bc+ ac. We can assume that a,b, and c are all greater than 1 since if (a,b,c) satisfies the inequality, then so does (ax,bx,cx) for any positive x. Using the fact that x^3 \geq x^2 if x>1, it is therefore sufficient to show that
a^2+b^2+c^2 \geq ab + bc +ac

Get everything over to the left hand side and multiply by 2. You will see that it factors nicely.

Edit: Oops, disregard what I said, I made a mistake in my algebra.
 
Last edited:

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