A petite observation about logarithms

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    Logarithms Observation
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Discussion Overview

The discussion revolves around the properties of logarithms, specifically the expressions ln(x^2) and 2ln(x). Participants explore the implications of these expressions in different contexts, particularly regarding the treatment of negative values of x and the use of absolute values in logarithmic identities.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that ln(x^2) and 2ln(x) appear different when considering the positive root in the logarithmic property ln(x^a) = a ln(x).
  • Another participant points out that for x < 0, ln(x^2) is real, while 2ln(x) has a non-zero imaginary part, indicating they cannot be equal.
  • Some participants agree that it is more appropriate to express ln(x^2) as 2ln(|x|) to avoid confusion, especially when solving equations.
  • One participant mentions that in programming contexts, using ln(x^2) instead of 2ln(|x|) may not be advisable.
  • A later reply clarifies that the original point was about preferring the expression ln(x^2) = 2ln(|x|) over ln(x^2) = 2ln(x), not about the general preference for one form over the other.

Areas of Agreement / Disagreement

Participants generally agree on the importance of using absolute values in the logarithmic expression to avoid issues with negative inputs, but there is some disagreement regarding the relevance of programming practices and the interpretation of the original point made.

Contextual Notes

The discussion highlights the nuances in handling logarithmic expressions, particularly in relation to the domain of x and the implications of using absolute values. There are unresolved aspects regarding the best practices in programming versus mathematical notation.

homology
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Here's something cute:

Consider the graph of ln(x^2) and then consider the graph of 2ln(x), missing anything?

I was momentarily caught off guard by this until I realized that when we derive the property: ln(x^a)=aln(x), we choose the positive root.

Has anyone ever run into a situation where it was better to say that ln(x^2)=2ln|x| ?
 
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Well, consider the case x < 0. Then ln(x^2) is real, but 2ln(x) has a non-zero imaginary part, so clearly they can't be equal.
 
Yes, that's a very good point.

Solving the equation ln(x^2)= 0 is NOT the same as solving 2ln(x)= 0 and, yes, it is better to write ln(x^2)= 2ln(|x|).
 
HallsofIvy said:
Yes, that's a very good point.
Solving the equation ln(x^2)= 0 is NOT the same as solving 2ln(x)= 0 and, yes, it is better to write ln(x^2)= 2ln(|x|).

On most computers it is not better to write ln(x^2) instead of 2ln(|x|) in a computer program.
 
D H said:
On most computers it is not better to write ln(x^2) instead of 2ln(|x|) in a computer program.

I'm sorry, what does that have to do with my response? My point was NOT that it was better to write ln(x^2) rather than 2ln(|x|) but rather that it was better to write ln(x^2)= 2ln(|x|) rather than ln(x^2)= 2ln(x).
 

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