For every rational number, there exists sum of two irrational numbers

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Homework Help Overview

The discussion revolves around proving that for every rational number, there exist irrational numbers whose sum equals that rational number. The subject area is primarily focused on number theory and properties of rational and irrational numbers.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the possibility of using contraposition to approach the proof, questioning the validity of their formulations. There are suggestions to rewrite the statement to facilitate the use of contraposition. Additionally, some participants propose a "guess and check" method to find suitable irrational numbers.

Discussion Status

The discussion is active, with participants sharing their thoughts on different approaches. Some guidance has been offered regarding the use of specific examples to understand the problem better, and there is an acknowledgment of the simplicity of a proposed solution, though it has not been fully explored or agreed upon by all participants.

Contextual Notes

Participants note the challenge of generalizing the sum of two irrational numbers given the universal quantifier in the original statement. There is also a recognition of the need to clarify definitions and assumptions regarding rational and irrational numbers.

ckwn87
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Homework Statement



Prove: For every rational number z, there exists irrational numbers x and y such that x + y = z.

Homework Equations



by definition, a rational number can be represented by ratio of two integers, p/q.

The Attempt at a Solution



Is there a way to do this by contraposition?

Would the contraposition be, For all rational numbers x and y, there does not exist an irrational number z such that x + y = z? I can handle from there, but I don't think my contraposition is correct.
 
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ckwn87 said:
Is there a way to do this by contraposition?
The question, as stated doesn't have any "if ... then ..." clauses in it. So, you have to rewrite it. You could, for example, sue
For every real z, if (z is rational) then (there exists irrational x and y such that x+y=z​
which you could contrapositive. I'm not sure it helps, though.


Have you tried one the simplest of all techniques -- guess and check?
 
Hurkyl said:
Have you tried one the simplest of all techniques -- guess and check?


Hmm, you're right, I don't think contraposition will help. I'm not sure what you mean by guess and check. Since it says for ALL Z, I'm not sure how I would generalize a sum of two irrational numbers.
 
You can guess for the existentials, though.

(And you can always try specific z's to get an idea before tackling the universal case)
 
Prove: For every rational number z, there exists irrational numbers x and y such that x + y = z.
Proof: x=z/2+sqrt(2), y=z/2-sqrt(2), so x+y=z and x, and y are irrational.
 
It looks that simple dimitri.
 
It is, isn't it?
 

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