Log, or logarithmic, functions undo or are the inverse function of power or exponential functions. The expression log_{b}x means the power to which b must be raised to in order to get x.
On Log Functions 

See below how the x and y values for the log and exponential are interchanged.


Another way to look at the inverse relationship is to see how one function is the mirror image of the other  x exchanges with y  for the natural log and the exponential with base e. You may need to hit REFRESH or RELOAD on your browser in order for the animation to be visible.
The slope of log_{b}x is (log_{b}e)(1/x) where x is greater than zero.
The inverse function is y = b^{x} .
Though any positive base may be used for a logarithm, the bases 10 and e are those most frequently used. Log_{10} x is called the common log. Log_{e} x or ln x is called the natural log.
If a log with a base other than 10 or e is needed, it may be estimated or the following statement may be employed, often where c is either 10 or e.
log_{b}a =(log_{c}a)/(log_{c}b)
The natural log function is the inverse functions of the exponential function, e^{x}. The expression ln x means the power to which e must be raised to in order to get x. For example: Log_{e} x has values of 0, 1, 2, 3, and 1/2 for x values of 1, e, e^{2}, e^{3}, and e^{.5}.
The slope of ln x is 1/x where x is greater than zero. The inverse function is y = e^{x} .
This page is from Exploring Functions Throught the Use of Manipulatives (ISBN: 0962359335).
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