The log function

The log function

Background: Any positive number, y, can be written as 10 raised to some power, x. We can write this relationship in equation form:

y = 10 ^x

For example it is obvious that 1000 can be written as 10 ³. It is not so obvious that 16 can be written as 10 ^1.2. How do we know that this is the correct power of 10? Because we get it from the graph shown below.

To make this graph we made a table of a few obvious values of  y = 10 ^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 = 16 and x = 1.2 (the black dot). This means that 16 = 10 ^1.2.

We next define a function called the logarithm that takes a number like 16 as input, calculates that it can be written as 10 ^1.2, and returns the exponent 1.2 as its output value. Here is the formal definition.

Example 1:   Evaluate log ( 10 ^5.7 ). In this example the argument of the log function is already expressed as 10 raised to an exponent, so the log function simply returns the exponent:

log ( 10 ^5.7 ) = 5.7

Example 2:   Evaluate log ( 1000 ). The argument is a number that is easily expressed as 10 raised to an exponent. We do this and the log function then returns the exponent:

log ( 1000 ) = log ( 10 ³ ) = 3

Example 3:   Evaluate log ( 16 ). The argument is a number which we don't know how to express as 10 raised to an exponent (unless we remember the above discussion which said that 16 = 10 ^1.2 ). Therefore we use a calculator or the Algebra Coach to evaluate it:

log ( 16 ) = 1.2

Example 4:   log ( x ). The argument is an expression. Until we can evaluate that expression we must leave this logarithm as is.

Graph: The blue curve shown to the right is the graph of the log function. Notice that for any positive x it returns a single value. For any negative x it is undefined. If you compare this graph of the log function to thegraph of y = 10 ^x then you see that one can be gotten from the other by interchanging the x and y axes.

For comparison we have also shown the graph of the ln function (the natural log function) in red. The ln graph has the same shape as the log graph but is 2.3 times as tall. The ln function is described below.

An important feature of the log function is that it increases very slowly as x becomes very large. It describes nicely how the human ear percieves loud sounds and the way the human eye percieves bright lights.



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

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

Some special values of the log function

Solving the Equation 10 ^x = y for x by using the log function

Suppose that x is unknown but that 10 ^x equals a known value y. Then finding x requires solving the following equation for x:

10 ^x = y

The solution is:

x = log (y)

This is because finding log (y) means expressing y as 10 to an exponent, and then returning that exponent. But the original equation says that that exponent is x. Note that if y is negative then there is no real solution. However there is a complex solution. Furthermore if y = 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 log function is defined to do exactly the opposite, namely:

Therefore these are inverse functions.

Note the following:

• Because the 10 ^x function is the inverse of the log function it is sometimes called the antilog function. 
• We saw above that the solution of 10 ^x = y  is  x = log (y). We should look at these two equations as expressing the same relationship between x and ybut from different points of view. The first equation is the relationship solved for y and the second one is the relationship solved for x. (An analogy is that the statement “Tom is Jane’s brother” is equivalent to the statement that “Jane is Tom’s sister”.) 
• In the previous bullet we saw that the two equations, 10 ^x = y  and  x = log (y), said the same thing. If we replace x in the first equation by the x of the second equation we get this identity:

10 ^log( y) = y

and if we replace y in the second equation by the y of the first equation we get this identity:

x = log (10 ^x )

These identities are useful for showing how the log and antilog functions cancel each other.

• If you compare the graph of y = log (x) to the graph of y = 10 ^x then you see that one can be gotten from the other by interchanging the x and y axes. This always happens with inverse functions.

How to use the log function in the Algebra Coach

• Type log(x) into the textbox, where x is the argument. The argument must be enclosed in brackets. 
• Set the relevant options:
• Set the exact / floating point option. In floating point mode the log of any number is evaluated. In exact mode the log of an integer is not evaluated because doing so would result in an approximate number.
• Turn on complex numbers if you want to be able to evaluate the log of a negative or complex number.
• Click the Simplify button. 

Algorithm for the log function

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

The ln function

Background: You might find it useful to read the previous section on the log function before reading this section. Recall that the log 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 they use base 8!

In fact probably the most important number in all of mathematics (click here to see why) is the number 2.7182818284590…, 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. 

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.

From now on, to avoid confusion, we will call ln(x) the natural logarithm function and log(x) the base 10 logarithm function.


Example 1:   Evaluate ln ( e ^4.7 ). In this example the argument of the ln function is already expressed as e raised to an exponent, so the ln function simply returns the exponent:

ln ( e ^4.7 ) = 4.7

Example 2:   Evaluate ln ( 8.6 ). The argument is a number which we don't know how to express as e raised to an exponent . Therefore we use a calculator or the Algebra Coach to evaluate it:

ln ( 8.6 ) = 2.15

Example 3:   ln ( x ). The argument is an expression. Until we can evaluate that expression we must leave this natural logarithm as is. 


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

ln ( e ) = ln ( e ¹ ) = 1

Example 5:   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 ln function. Notice that for any positive x it returns a single value. For any negative x it is undefined. If you compare this graph of the ln function to thegraph of y = e ^x then you see that one can be gotten from the other by interchanging the x and y axes.

For comparison we have also shown the graph of the base 10 log function in blue. The log graph has the same shape as the ln graph but is only 43% as tall. 



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

The ln 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 = y for x by using the ln function

Suppose that x is unknown but that e ^x equals a known value y. Then finding x requires solving this equation for x:

e ^x = y

The solution is:

x = ln (y)

because finding ln (y) means expressing y as e to an exponent, and then returning that exponent. But the original equation says that that exponent is x. Note that if y is negative then there is no real solution. However there is a complex solution. Furthermore if y = 0 then there is no solution at all. 

ln (x