Sums and Products of Rational and Irrational Numbers

Click For Summary

Discussion Overview

The discussion centers around the properties of sums and products involving rational and irrational numbers. Participants explore the implications of these operations, particularly focusing on whether the results remain within the set of rational numbers or not. The scope includes theoretical reasoning and examples to illustrate points made.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant explains that the sum, difference, and product of rational numbers yield rational numbers, citing closure properties.
  • Another participant questions whether the product of irrational numbers is necessarily irrational and seeks clarification on the sum of irrational numbers.
  • A participant provides an example showing that the sum of two irrational numbers can be rational, using \(\alpha=\pi\) and \(\beta=1-\pi\).
  • In response, another participant asserts that this does not imply that the sum of two irrational numbers is always rational, providing an example of \(\alpha=\sqrt{2}\) and \(\beta=e\), which results in an irrational sum.
  • Further, a participant cites \(\pi + \pi = 2\pi\) as another case where the sum of two irrational numbers remains irrational.
  • There is a request for a general rule regarding the sums of irrational numbers.
  • A participant introduces the concepts of existential and universal quantifiers in the context of proving mathematical statements.

Areas of Agreement / Disagreement

Participants express differing views on whether the sum of two irrational numbers can be rational, with examples provided that illustrate both possibilities. There is no consensus on a general rule regarding the sums of irrational numbers.

Contextual Notes

Participants discuss specific cases and examples but do not arrive at a universally applicable conclusion. The discussion reflects various assumptions about the nature of rational and irrational numbers without resolving the complexities involved.

paulmdrdo1
Messages
382
Reaction score
0
Explain why the sum, the difference, and the product of the
rational numbers are rational numbers. Is the product of the
irrational numbers necessarily irrational? What about
the sum?

Combining Rational Numbers with Irrational Numbers
In general, what can you say about the sum of a rational
and an irrational number? What about the product?

please explain with examples.
 
Physics news on Phys.org
Have you tried anything yourself yet? Here are a couple of hints...

What do you know about the closure of the rational numbers under addition and multiplication?

As for irrationals, what is \displaystyle \begin{align*} \sqrt{2} \cdot \sqrt{2} \end{align*}?
 
this is what i tried,

When we add two rational numbers say 1/3+2/3 = 3/3 = 1. 1 is an element of the set of rational numbers. therefore whenever addition is performed on the elements of the set of rational numbers, an element of the set is obtained. performing multiplication on the set is analogous to that of addition.
 
Last edited:
The sum of two irrational numbers can be rational, consider:

$$\alpha=\pi$$

$$\beta=1-\pi$$

$$\alpha+\beta=1$$
 
does that mean when we add two irrational number we always obtain rational numbers?
 
paulmdrdo said:
does that mean when we add two irrational number we always obtain rational numbers?

No, not by any means, I just demonstrated a case where the sum is rational. How about:

$$\alpha=\sqrt{2}$$

$$\beta=e$$

$$\alpha+\beta=\sqrt{2}+e$$

This sum is irrational.
 
paulmdrdo said:
does that mean when we add two irrational number we always obtain rational numbers?

Nope, not always. Consider $\pi + \pi=2\pi$. Two irrational numbers that add up to another irrational.
 
can you give me general rule about this.
 
Do you know anything about existentials and universals?

If you want to prove something is always (universally) true, you need to make an argument that shows that there is does not exist a situation where it is not true.

If you want to prove something is not always (not universally) true, you just need to show that there exists a situation where it is not true.
 

Similar threads

Replies
4
Views
3K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 17 ·
Replies
17
Views
7K
Replies
8
Views
2K
  • · Replies 13 ·
Replies
13
Views
4K
  • · Replies 30 ·
2
Replies
30
Views
3K
  • · Replies 41 ·
2
Replies
41
Views
5K
  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 1 ·
Replies
1
Views
1K