Proof: Help with Algebra: a^2 (1 + b^4) + b^2(1 + a^4)

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Discussion Overview

The discussion revolves around proving the inequality a^2 (1 + b^4) + b^2(1 + a^4) ≤ (1 + a^4)(1 + b^4). Participants explore various approaches to the proof, including factoring and rearranging terms, while examining the conditions under which the inequality holds.

Discussion Character

  • Mathematical reasoning
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant proposes starting with the expression a^2 (1 + b^4) + b^2(1 + a^4) and factoring it to (a^2 + b^2)(1 + a^2b^2), but expresses uncertainty about the next steps.
  • Another participant suggests examining the expression (1-a^2)^2(1+b^4)+(1-b^2)^2(1+a^4) and questions its implications, indicating a need for clarification on how it relates to the original inequality.
  • A participant mentions a method of proving inequalities by rearranging terms to show that they are non-negative, emphasizing the importance of expressing everything in terms of squares of real numbers.
  • There is a suggestion that if a rearrangement cannot be made, additional information about the variables a and b may be necessary for proving the inequality.
  • One participant expresses confusion about how a specific expression was derived, indicating a need for further explanation of the steps taken in the proof process.
  • Another participant provides a detailed breakdown of manipulating the original inequality to facilitate the proof, including multiplying by a positive number and collecting like terms.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the proof of the inequality, with multiple approaches and some confusion regarding specific expressions and steps in the reasoning process. The discussion remains unresolved regarding the best method to prove the inequality.

Contextual Notes

Some participants express uncertainty about the assumptions regarding the variables a and b, particularly whether they are real numbers, and how this affects the validity of the proposed approaches.

recon
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How can you proof that

[tex]a^2 (1 + b^4) + b^2(1 + a^4) \leq (1 + a^4)(1 + b^4)[/tex]?

I factorised [tex]a^2 (1 + b^4) + b^2(1 + a^4)[/tex] to [tex](a^2 + b^2)(1+a^2b^2)[/tex], but I don't really know where to go from here.
 
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Assuming a and b are real, what do you know must be true about the following expression?

[tex](1-a^2)^2(1+b^4)+(1-b^2)^2(1+a^4)[/tex]
 
I get it now. Thanks!

BTW, does anyone know a good site that teaches you factoring 'tricks'?
 
This isn't a factoring trick, but just a common observation that if you want to show

X > Y

then this is the same as showing

X-Y > 0

and what do we know is always positive?
 
PBRMEASAP said:
Assuming a and b are real, what do you know must be true about the following expression?

[tex](1-a^2)^2(1+b^4)+(1-b^2)^2(1+a^4)[/tex]

I don't see how you got this.
 
Divine inspiration, perhaps?
 
erraticimpulse said:
I don't see how you got this.

The general procedure is:

Take the original inequality that you are trying to prove and move everything over to one side, so that you have (stuff) >= 0. Now rearrange (stuff) so that it consists entirely of squares of real numbers, added and multiplied together. Since such an expression is automatically greater than or equal to zero, the original inequality must be true.

If it turns out that such a rearrangement can't be made, then you can't prove the inequality without more information about a and b, such as a > b or something similar. But in this example it was possible.
 
I think you misundestood me. It's not the concept I don't understand, just the result. I don't see how you got to that expression.
 
Oops, sorry about that. Start with the expression that we want to show is greater than or equal to zero.


[tex](1+a^4)(1+b^4)-a^2(1+b^4)-b^2(1+a^4)[/tex]

Since we are showing this is >= zero, it doesn't change anything to multiply the expression by a positive number, namely 2.

[tex]2(1+a^4)(1+b^4)-2a^2(1+b^4)-2b^2(1+a^4)[/tex]

Split up the first term:

[tex](1+a^4)(1+b^4)-2a^2(1+b^4)+(1+a^4)(1+b^4)-2b^2(1+a^4)[/tex]

Now collect terms 1 and 2 together, and 3 and 4 together, and you end up with the expression you are looking for.
 

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