But since distance is always positive, can we say that | a - b | = | b - a |?

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Discussion Overview

The discussion centers around the property of absolute values in the context of real numbers, specifically examining whether the equation |a - b| = |b - a| can be justified based on the concept of distance being always positive. The scope includes mathematical reasoning and conceptual clarification.

Discussion Character

  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant references a textbook stating that the distance between a and b is given by |a - b| = |b - a|, questioning if this can be derived from the property of distance being positive.
  • Another participant suggests proving the equality |a - b| = |b - a| using the definition of absolute value as |x| ≡ √(x²).
  • A participant provides a specific example with a = 4 and b = 3 to illustrate that |4 - 3| = |3 - 4| results in 1 = 1, questioning if this is a valid proof.
  • Another participant critiques the use of fixed values as insufficient for a general proof, reiterating the definition of absolute value and suggesting a more formal approach using square roots.
  • A participant expresses their limitations in proving concepts beyond specific examples, sharing a personal interpretation of the absolute value definition through a metaphorical example.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the sufficiency of using specific numerical examples as proof. There is a general agreement that a more formal proof is necessary, but no definitive resolution is presented regarding the validity of the claims made.

Contextual Notes

Participants express uncertainty about the adequacy of their mathematical reasoning and the generalizability of their examples. The discussion reflects a range of mathematical understanding and approaches to proof.

mathdad
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On page 9 of my David Cohen Precalculus textbook (3rd Edition), the following property is given:

The distance between a and b is | a - b | = | b - a |.

Question:

Can we say that | a - b | = | b - a | because distance is always positive?

Note: a and b are real numbers.
 
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Given that:

$$|x|\equiv\sqrt{x^2}$$

Can you prove that:

$$|a-b|=|b-a|$$ ?
 
MarkFL said:
Given that:

$$|x|\equiv\sqrt{x^2}$$

Can you prove that:

$$|a-b|=|b-a|$$ ?

If a and b are real numbers, I can plug any for a and b.

Let a = 4 and b = 3.

| 4 - 3| = | 3 - 4 |

| 1 | = | -1 |

1 = 1

True?
 
RTCNTC said:
If a and b are real numbers, I can plug any for a and b.

Let a = 4 and b = 3.

| 4 - 3| = | 3 - 4 |

| 1 | = | -1 |

1 = 1

True?

Generally, using fixed values doesn't constitute a good proof that will hold for all values. Using the definition I gave, we can write:

$$\sqrt{(a-b)^2}=\sqrt{(b-a)^2}$$

Can you show this must be true?
 
MarkFL said:
Generally, using fixed values doesn't constitute a good proof that will hold for all values. Using the definition I gave, we can write:

$$\sqrt{(a-b)^2}=\sqrt{(b-a)^2}$$

Can you show this must be true?

I cannot prove anything outside of fixed values at my level of math.

I know that the square root of a square yields the radicand.

I like to look at it this way:

sqrt{(cars - road)^2} = cars - road.

sqrt{(road - cars)^2} = road - cars.
 
Last edited:

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