Is the rewritten form of ln(x2) valid?

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Discussion Overview

The discussion revolves around the validity of rewriting the logarithmic function ln(x²) and the implications of domain restrictions when performing such transformations. It touches on calculus concepts related to logarithmic properties and their domains.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the validity of the rewritten form of ln(x²) and its domain implications.
  • Another participant states that ln(x²) can be rewritten as 2 ln(|x|), citing the relationship between the square root and absolute value.
  • A subsequent reply acknowledges the correction regarding the rewriting of ln(x²).
  • Another participant points out that ln(x²) is defined for all real x ≠ 0, while 2 ln(x) is only defined for x > 0, emphasizing the importance of domain in logarithmic functions.
  • This participant also notes that logarithmic properties have limitations based on the positivity of their arguments, highlighting that ln(a*b) can be defined even if the right side is undefined under certain conditions.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the rewritten form and the implications of domain restrictions. There is no consensus on the overall validity of the transformation without further clarification on the domains involved.

Contextual Notes

Limitations include the need for careful consideration of the domains of the logarithmic functions involved and the potential for undefined expressions based on the values of x.

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I'm reading back over a calculus book getting ready for an exam and I'm seeing a note that I don't understand.

It says to make sure, when rewriting a ln function that the domain is the same, then it provides an example of when it's not the same, yet says nothing more. Is this rewritten form valid?

https://dl.dropbox.com/u/15809883/ln.png
 
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ln(x^2) = 2 ln(|x|)

As sqrt(x^2)=|x|
 
Ah yes, I had forgotten about that, thank you.
 
The difference in the graphs is entirely due to the domains of the two different functions.

ln(x2) is defined for all real x ≠ 0.
2 ln(x) is defined only for x > 0.

The rules for logarithms contain limitations on the values of the arguments. For example, ln(a*b) = ln(a) + ln(b), where a > 0 and b > 0. Note that it is possible for ln(a * b) to be defined even though the right side is undefined. This can happen when both a and b are negative.
 

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