base 10 logarithm (log)     natural logarithm (ln)     exponential function (exp or ex )  

 

The base 10 logarithm function

Background: Every positive number, y, can be expressed as 10 raised to some power, x. This relationship is described by the equation
y = 10 x,
and described by this graph:
For example the number 16 can be expressed as 10 1.2. This is the black dot in the graph. We define a function called the base 10 logarithm that takes a number like 16 as input, calculates that it can be expressed as 10 1.2, and returns the exponent 1.2 as its output value:
Here is the formal definition of the base 10 logarithm function.


Definition: The base 10 logarithm is the function that takes any positive number x as input and returns the exponent to which the base 10 must be raised to obtain x. It is denoted log(x).




Example 1:   Evaluate log ( 1000 ).

The argument of the logarithm function (i.e. the quantity in brackets, 1000), is easily expressed as 10 raised to the exponent 3. The logarithm function then returns the exponent.
log ( 1000 ) = log ( 10 3 ) = 3

Example 2:   Evaluate log ( 10 5.7 ).

The argument is already expressed as 10 raised to an exponent, so the logarithm function simply returns the exponent.
log ( 10 5.7 ) = 5.7

Example 3:   Evaluate log ( 16 ).

According to the graph 16 = 10 1.2. The logarithm function returns the exponent 1.2.
log ( 16 ) = log ( 10 1.2 ) = 1.2




Graph: The blue curve shown to the right is the graph of the base 10 logarithm function, y = log(x). Notice that for any positive x it is single valued and for any negative x it is undefined. If you compare this graph to the graph of y = 10 x above then you see that one can be gotten from the other by interchanging the x and y axes.

For comparison the red curve is the graph of the natural logarithm function (y = ln(x), covered in the next section). The natural logarithm graph has exactly the same shape as the base 10 logarithm graph; it is just 2.3 times as tall.

An important feature of logarithm functions (no matter what base) is that they increase very slowly as x becomes very large. They describe nicely how the human ear perceives loudness and the way the human eye perceives brightness.



Domain and range: The domain of the base 10 logarithm function is all positive real numbers and the range is all real numbers.

The base 10 logarithm function can be extended to the complex numbers, in which case the domain is all complex numbers except zero. The base 10 logarithm of zero is always undefined.




Some special values of the base 10 logarithm function





Solving the equation 10 x = c for x by using the base 10 logarithm function

Suppose that x is unknown but that 10 x equals a known value c. Then finding x requires solving the following equation for x.
10 x = c
The solution is
x = log (c)
This is because finding log (c) means expressing c as 10 to an exponent, and then returning that exponent. But the original equation says that that exponent is x. Note that if c is negative then there is no real solution. However there is a complex solution. Furthermore if c = 0 then there is no solution at all.




log (x) and 10 x are inverse functions

Consider the 10 x function which takes x and returns 10 x, like this:
The base 10 logarithm function is defined to do exactly the opposite, namely:
Therefore these are inverse functions.

Note the following:


How to use the base 10 logarithm function in the Algebra Coach


Algorithm for the base 10 logarithm function

Click here to see the algorithm that computers use to evaluate the base 10 logarithm function.




The natural logarithm function

Background: You might find it useful to read the previous section on the base 10 logarithm function before reading this section. The two sections closely parallel each other.

Recall that the base 10 logarithm function takes a number like 16 as input, calculates that it can be written as 10 1.2, and then returns the exponent 1.2 as its output value. But why use base 10? After all, probably the only reason that the number 10 is important to humans is that they have 10 fingers with which they first learned to count. Maybe on some other planet populated by 8-fingered beings they use base 8!

In fact probably the most important number in all of mathematics (click here to see why) is the number 2.71828…, which we give the name e, in honor of Leonard Euler, who first discovered it. It will be important to be able to take any positive number, y, and express it as e raised to some power, x. We can write this relationship in equation form:
y = e x
For example 5 can be written as e 1.6 (the exponent is approximate). How do we know that this is the correct power of e? Because we get it from the graph shown below.

To make this graph we made a table of a few obvious values of  y = e x  as shown below, left. Then we plotted the values in the graph (they are the red dots) and drew a smooth curve through them. Then we observed that the curve went through y = 5 and x = 1.6 (the black dot). This means that 5 = e 1.6.


21

If you compare this graph to that of y = 10 x you see that both have the same so-called exponential growth shape but that this graph grows more slowly.



We next define a function called the natural logarithm that takes a number like 5 as input, calculates that it can be written as e 1.6, and returns the exponent 1.6 as its output value. Here is the formal definition.


Definition: The natural logarithm is the function that takes any positive number x as input and returns the exponent to which the base e must be raised to obtain x. It is denoted ln(x). (e denotes the number 2.71828…)


Note that to avoid confusion the natural logarithm function is denoted ln(x) and the base 10 logarithm function is denoted log(x) .


Example 1:   Evaluate ln ( e 4.7 ).

The argument of the natural logarithm function is already expressed as e raised to an exponent, so the natural logarithm function simply returns the exponent.
ln ( e 4.7 ) = 4.7

Example 2:   Evaluate ln ( 5 ).

According to the graph 5 = e 1.6. The logarithm function returns the exponent 1.6.
ln ( 5 ) = 1.6
 
Example 3:   Evaluate ln ( e ).

Express the argument as e raised to the exponent 1 and return the exponent.
ln ( e ) = ln ( e 1 ) = 1
 
Example 4:   Evaluate ln ( 1 ).

Express the argument as e raised to the exponent 0 and return the exponent.
ln ( 1 ) = ln ( e 0 ) = 0




Graph: The red curve shown to the right is the graph of the natural logarithm function, y = ln (x). Notice that for any positive x it is single valued and for any negative x it is undefined. If you compare this graph to the graph of y = e x then you see that one can be gotten from the other by interchanging the x and y axes.

For comparison the blue curve shows the base 10 logarithm function, y = log (x). It has exactly the same shape but is only 43% as tall.



Domain and range: The domain of the natural logarithm function is all positive real numbers and the range is all real numbers.

The natural logarithm function can be extended to the complex numbers, in which case the domain is all complex numbers except zero. The natural logarithm of zero is always undefined.




Solving the Equation e x = c for x by using the natural logarithm function

Suppose that x is unknown but that e x equals a known value c. Then finding x requires solving this equation for x:
e x = c
The solution is
x = ln (c)
because finding ln (c) means expressing c as e raised to an exponent, and then returning that exponent. But the original equation says that that exponent is x. Note that if c is negative then there is no real solution. However there is a complex solution. Furthermore if c = 0 then there is no solution at all.




ln (x) and e x are inverse functions

Consider the e x function which takes x and returns e x, like this:
The natural logarithm function is defined to do exactly the opposite, namely:
Therefore these are inverse functions.

Note the following:


How to use the natural logarithm function in the Algebra Coach


Algorithm for the natural logarithm function

Click here to see the algorithm that computers use to evaluate the natural logarithm function.




The exponential function (with base e, the e x function)

Background: You might find it useful to read the previous section on the natural logarithm function before reading this section. There we saw that it is possible to use the number e (which is approximately 2.71828…) as a base and to raise it to any power, x, and produce any positive number y. We can write this relationship in equation form:
y = e x
Here is a graph of y = e x (the blue curve). For comparison we also show graphs of y = 2 x and y = 4 x. Because the number e is between 2 and 4 the curve y = e x lies between the curves y = 2 x and y = 4 x.



All three of these curves are called exponential functions because the independent variable x is in the exponent. All three have the property that the higher up the curve you go the steeper they get. However y = e x has the special property that at every point along the curve the slope equals the height. This property concerning slopes makes it a very important function in calculus.

Note: In this section when we say ‘the exponential function’ we mean the one with base e.



Definition: We define a function called the exponential function (denoted exp) that takes an argument x and returns the value of e raised to the power x:

You can think of exp(x) as just an alternative (functional) notation for the expression e x. So of course the functional form exp(x) has all the properties that the exponential form e x has. The Algebra Coach has an option that allows you to use one form or the other. Here is a table comparing the “look” of the various properties in the two forms:
Functional form Exponential form
info on this property
info on this property
info on this property
info on this property
info on this property
info on this property
info on this property
info on this property




Graph: The blue curve is the graph of  y = e x  (i.e. of the exponential function). It has the property that its slope equals its height everywhere. The dotted red lines show the slope of the curve at various points along the curve. Notice that the slope is 5 when the height is 5, and so on.

If you compare this graph of the exponential function to the graph of the natural logarithm function then you see that one can be gotten from the other by interchanging the x and y axes.

Domain and range: The domain of the exponential function is all real numbers and the range is all positive real numbers.

The exponential function can be extended to the complex numbers, in which case the domain and the range is all complex numbers.



Solving the Equation ln (x) = c for c by using the exponential function

Suppose that x is unknown but that ln (x) equals a known value c. Then finding x requires solving this equation for x:
ln (x) = c.
The solution is:
x = e c,    or    x = exp (c).
This was explained in the previous section on the natural logarithm function.




ln (x) and e x are inverse functions

This was explained in the previous section on the natural logarithm function.



How to use the exponential function in the Algebra Coach Note: You never need to use the functional notation exp (x) in the Algebra Coach. You can always use the notation e x instead. In fact the default setting for the exponential function option is exp (x) → e x.




Algorithm for the e x or exponential function

Click here to see the algorithm that computers use to evaluate e x or exp (x).





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