Convert Between Logarithmic and Exponential

Logarithms are the inverses of exponential functions. You will explore the relationship between an exponential and a logarithmic function. You will also explore the basic characteristics of a logarithmic function, including domain, range, and long-run behavior.

Converting from Logarithmic to Exponential Form

In order to analyze the magnitude of earthquakes or compare the magnitudes of two different earthquakes, we need to be able to convert between logarithmic and exponential form. For example, suppose the amount of energy released from one earthquake were 500 times greater than the amount of energy released from another. We want to calculate the difference in magnitude. The equation that represents this problem is $10^x=500$ , where $x$ represents the difference in magnitudes on the Richter Scale. How would we solve for $x$ ?

We have not yet learned a method for solving exponential equations. None of the algebraic tools discussed so far is sufficient to solve $10^x=500$ . We know that $10^2=100$ and $10^3=1000$ , so it is clear that $x$ must be some value between 2 and 3, since $y=10^x$ is increasing. We can examine a graph, as in Figure 2, to better estimate the solution.

Figure 2

Estimating from a graph, however, is imprecise. To find an algebraic solution, we must introduce a new function. Observe that the graph in Figure 2 passes the horizontal line test. The exponential function $y=b^x$ is one-to-one, so its inverse, $x=b^y$ is also a function. As is the case with all inverse functions, we simply interchange $x$ and $y$ and solve for $y$ to find the inverse function. To represent $y$ as a function of $x$ , we use a logarithmic function of the form $y=log_b(x)$ . The base $b$ logarithm of a number is the exponent by which we must raise $b$ to get that number.

We read a logarithmic expression as, "The logarithm with base $b$ of $x$ is equal to $y$ , " or, simplified, "log base $b$ of $x$ is $y$ ". We can also say, " $b$ raised to the power of $y$ is $x$ , " because logs are exponents. For example, the base 2 logarithm of 32 is 5, because 5 is the exponent we must apply to 2 to get 32. Since $2^5=32$ , we can write $log_232=5$ . We read this as "log base 2 of 32 is 5".

We can express the relationship between logarithmic form and its corresponding exponential form as follows:

$\log _{b}(x)=y \Leftrightarrow b^{y}=x, b > 0, b \neq 1$

Note that the base $b$ is always positive.

Because logarithm is a function, it is most correctly written as $log_b(x)$ , using parentheses to denote function evaluation, just as we would with $f(x)$ . However, when the input is a single variable or number, it is common to see the parentheses dropped and the expression written without parentheses, as $log_bx$ . Note that many calculators require parentheses around the $x$ .

We can illustrate the notation of logarithms as follows:

Notice that, comparing the logarithm function and the exponential function, the input and the output are switched. This means $y=log_b(x)$ and $y=b^x$ are inverse functions.

DEFINITION OF THE LOGARITHMIC FUNCTION

A logarithm base $b$ of a positive number $x$ satisfies the following definition.

For $x > 0,b > 0,b \neq 1$ ,

$y=log_b(x) \quad$ is equivalent to $b^y=x$

where,

we read $log_b(x)$ as, "the logarithm with base $b$ of $x$ " or the "log base $b of x$ ".
the logarithm $y$ is the exponent to which $b$ must be raised to get $x$ .

Also, since the logarithmic and exponential functions switch the $x$ and $y$ values, the domain and range of the exponential function are interchanged for the logarithmic function. Therefore,

the domain of the logarithm function with base $b$ is $(0, \infty)$ .
the range of the logarithm function with base $b$ is $(− \infty, \infty)$ .

Q&A

Can we take the logarithm of a negative number?

No. Because the base of an exponential function is always positive, no power of that base can ever be negative. We can never take the logarithm of a negative number. Also, we cannot take the logarithm of zero. Calculators may output a log of a negative number when in complex mode, but the log of a negative number is not a real number.