Is My Proof That ab<a Correct Given 0<a<1 and 0<b<1?

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Homework Help Overview

The discussion revolves around a proof concerning the product of two positive numbers, \(a\) and \(b\), both constrained between 0 and 1. Participants are examining the validity of the assertion that \(ab < a\) under these conditions.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the implications of multiplying inequalities and whether the inequality signs remain valid. There is also a consideration of the axioms and rules that govern the proof.

Discussion Status

Some participants have provided feedback on the original poster's proof, noting the importance of the positivity of \(a\) in maintaining the direction of the inequalities. Others have raised questions about the necessity of a final conclusion in the proof.

Contextual Notes

There is an ongoing exploration of the axioms and rules that may apply to the proof, indicating that the discussion is still in a formative stage with various interpretations being considered.

knowLittle
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I would like some feedback on my proof.

Can I just say that :
0< a<1 ... [1]
0<b<1 ... [2]

multiplying [2] by 'a' everywhere, then I get
0<ab<a

And, we prove that ab<a?
 
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Yes. But you need to note that the multiply does not reverse the inequality signs because "a" is always positive.
(and don't you want a final QED?)
 
knowLittle said:
I would like some feedback on my proof.

Can I just say that :
0< a<1 ... [1]
0<b<1 ... [2]

multiplying [2] by 'a' everywhere, then I get
0<ab<a

And, we prove that ab<a?
Actually, if 0 < a and 0 < b < 1, then ab < a. a does not have to be less than 1.

-Dan
 
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I don't want to be overkill on this, but the proof will depend on the axioms and rules you're going by.
You may, though, move the a to the other side and factor.
 
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Thank you all!
 
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