Is d/dx of ln(|x|) = 1/x? Proving the Claim

  • Context: Undergrad 
  • Thread starter Thread starter vikcool812
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Discussion Overview

The discussion centers around the differentiation of the natural logarithm of the absolute value function, specifically whether the derivative of ln(|x|) equals 1/x for x not equal to 0. The scope includes mathematical reasoning and technical explanation related to calculus.

Discussion Character

  • Technical explanation, Mathematical reasoning, Debate/contested

Main Points Raised

  • One participant states that the derivative of ln(x) is 1/x for x > 0 and questions the validity of the claim that d/dx of ln(|x|) is also 1/x for x not equal to 0.
  • Another participant suggests using the chain rule of differentiation to approach the problem.
  • A participant explains that ln(|x|) can be expressed as ln(x) for x > 0 and ln(-x) for x < 0, leading to the conclusion that d/dx(ln(|x|)) = 1/x in both cases.
  • One participant warns that the formula d/dx(ln(|x|)) = 1/x is incorrect for complex x.
  • Another participant emphasizes the importance of using the definition of absolute value when dealing with derivatives or limits.
  • A participant expresses gratitude for the suggestions provided in the discussion.
  • One participant acknowledges a hint received and clarifies that their work pertains to real numbers.

Areas of Agreement / Disagreement

There is no consensus on the applicability of the derivative formula for complex numbers, and the discussion includes multiple viewpoints regarding the differentiation of ln(|x|).

Contextual Notes

Participants reference the definitions of absolute values and the chain rule, but there are no explicit resolutions to the concerns raised about complex numbers or the general applicability of the derivative.

vikcool812
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I know d/dx of ln(x) is 1/x , x>0.
A website says d/dx of ln(|x|) is also 1/x for x not = 0 .
is that true , i am unable to prove it!
 
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Use the chain rule of differentiation.
 
Hi vikcool812 :
Note that ln(|x|)=ln(x) when x>0 and
=ln(-x) when x<0
So d/dx (ln(|x|))=d/dx(ln(x)) when x>0 and
d/dx (ln(|x|))=d/dx(ln(-x)) when x<0 therefore
d/dx(ln(x)) =1/x when x>0 by definition of ln(x) and
d/dx(ln(-x))=-1/-x = 1/x when x<0 by chain rule so in both cases we have
d/dx (ln(|x|))=1/x for not x=0
Best Regards
Riad Zaidan
 
BEWARE... The formula
\frac{d}{dx}\ln(|x|) = \frac{1}{x}
is wrong for complex x
 
Remember that when dealing with absolute values, with either derivatives or limits, it is best to directly use the definition of the absolute value. I.e.,
|x| = \begin{cases} x &amp;, x\geq 0 \\ -x &amp;, x\leq 0 \end{cases}
This is of course what rzaidan used in his solution.
 
Thank You ! Everyone for your valuable suggestions .
 
Hi g_edgar
THankyou for your hint , and my work was in the real numbers.
Best Regards
Riad Zaidan
 

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