# logarithmic differentiation

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• May 25th 2013, 07:40 PM
muddywaters
logarithmic differentiation
why do we have to rewrite as $|y|=|x|^2|3x-1|^3$ ?
• May 25th 2013, 09:01 PM
ibdutt
Re: logarithmic differentiation
Because log function is not defined for negative numbers
• May 25th 2013, 10:56 PM
muddywaters
Re: logarithmic differentiation
isnt it that $ln|x|=\frac{1}{x}$ both $lnx$ and $ln(-x)$ equals to $\frac{1}{x}$, so it doesn't matter?
and also, when it comes to rewriting using modulus, how do i know for sure the correct way to write it? why not $|y|=|x^5(3x-1)^3|$
or $|y|=|x|^5(3|x|-1)^3$
• May 26th 2013, 03:04 AM
ibdutt
Re: logarithmic differentiation
• May 26th 2013, 04:29 AM
muddywaters
Re: logarithmic differentiation
oh yeah thanks and sorry it should be $(ln|x|)'=\frac{1}{x}$
• May 26th 2013, 05:53 AM
HallsofIvy
Re: logarithmic differentiation
No, it should be $(ln|x|)'= \frac{1}{|x|}$.
• May 26th 2013, 06:33 AM
muddywaters
Re: logarithmic differentiation
Quote:

Originally Posted by HallsofIvy
No, it should be $(ln|x|)'= \frac{1}{|x|}$.

why do you say so?
• May 26th 2013, 06:35 AM
muddywaters
Re: logarithmic differentiation
i mean, is there some significance?? it seems the same to me in the end
• May 26th 2013, 06:47 AM
wondering
Re: logarithmic differentiation
Draw the graph of ln(x). It's derivative is always increasing so 1/x has to be positive for the domain of the original function. You can also draw the graph of 1/x. There is a positive and negative portion of the graph. The absolute value keeps only the positive portion. The first derivative tells you how the original function changes or how it's slope changes. As you get close to zero the slope gets really steep. Try putting really small positive numbers into 1/x. What happens?
• May 26th 2013, 07:24 AM
muddywaters
Re: logarithmic differentiation
Quote:

Originally Posted by wondering
Draw the graph of ln(x). It's derivative is always increasing so 1/x has to be positive for the domain of the original function. You can also draw the graph of 1/x. There is a positive and negative portion of the graph. The absolute value keeps only the positive portion. The first derivative tells you how the original function changes or how it's slope changes. As you get close to zero the slope gets really steep. Try putting really small positive numbers into 1/x. What happens?

?? u get a huge number approaching infinity? but how is that related to what we're discussing?
• May 26th 2013, 07:52 AM
Plato
Re: logarithmic differentiation
Quote:

Originally Posted by muddywaters
it should be $(\ln|x|)'=\frac{1}{x}$

That is correct. See here.
• May 26th 2013, 07:58 AM
wondering
Re: logarithmic differentiation
You get a really small number approaching infinity and a really big number as you approach zero. Does that match how the slope of ln(x) changes from (0,inf)?