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Course: Algebra 2 > Unit 8

Lesson 1: Introduction to logarithms

Intro to Logarithms

Learn what logarithms are and how to evaluate them.

What you should be familiar with before taking this lesson

You should be familiar with exponents, preferably including negative exponents.

What you will learn in this lesson

You will learn what logarithms are, and evaluate some basic logarithms. This will prepare you for future work with logarithm expressions and functions.

What is a logarithm?

Logarithms are another way of thinking about exponents.

For example, we know that

2

raised to the

4^{th}

power equals

16

. This is expressed by the exponential equation

2^{4} = 16

Now, suppose someone asked us, "

2

raised to which power equals

16

?" The answer would be

4

. This is expressed by the logarithmic equation

\log_{2} (16) = 4

, read as "log base two of sixteen is four".

2^{4} = 16 ⟺ \log_{2} (16) = 4

Both equations describe the same relationship between the numbers

2

4

, and

16

, where

2

is the base and

4

is the exponent.

The difference is that while the exponential form isolates the power,

16

, the logarithmic form isolates the exponent,

4

Here are more examples of equivalent logarithmic and exponential equations.

Logarithmic form		Exponential form
$\log_{2} (8) = 3$ ‍	$⟺$ ‍	$2^{3} = 8$ ‍
$\log_{3} (81) = 4$ ‍	$⟺$ ‍	$3^{4} = 81$ ‍
$\log_{5} (25) = 2$ ‍	$⟺$ ‍	$5^{2} = 25$ ‍

Definition of a logarithm

Generalizing the examples above leads us to the formal definition of a logarithm.

\log_{b} (a) = c ⟺ b^{c} = a

Both equations describe the same relationship between

a

b

, and

c

$b$ ‍ is the $base$ ‍,
$c$ ‍ is the $exponent$ ‍, and
$a$ ‍ is called the $argument$ ‍.

A helpful note

When rewriting an exponential equation in log form or a log equation in exponential form, it is helpful to remember that the base of the logarithm is the same as the base of the exponent.

Check your understanding

In the following problems, you will convert between exponential and logarithmic forms of equations.

Problem 1

Which of the following is equivalent to $2^{5} = 32$ ‍?

Problem 2

Which of the following is equivalent to $5^{3} = 125$ ‍?

Problem 3

Write $\log_{2} (64) = 6$ ‍ in exponential form.

Problem 4

4) Write $\log_{4} (16) = 2$ ‍ in exponential form.

Evaluating logarithms

Great! Now that we understand the relationship between exponents and logarithms, let's see if we can evaluate logarithms.

For example, let's evaluate

\log_{4} (64)

Let's start by setting that expression equal to

x

\log_{4} (64) = x

Writing this as an exponential equation gives us the following:

4^{x} = 64

4

to what power is

64

? Well,

4^{3} = 64

and so

\log_{4} (64) = 3

As you become more practiced, you may find yourself condensing a few of these steps and evaluating

\log_{4} (64)

just by asking, " $4$ ‍ to what power is $64$ ‍?"

Check your understanding

Remember, when evaluating

\log_{b} (a)

, you can ask: "

b

to what power is

a

Problem 5

\log_{6} (36) =

Problem 6

\log_{3} (27) =

Problem 7

\log_{4} (4) =

Problem 8

\log_{5} (1) =

Challenge problem

\log_{3} (\frac{1}{9}) =

Restrictions on the variables

\log_{b} (a)

is defined when the base

b

is positive—and not equal to

1

—and the argument

a

is positive. These restrictions are a result of the connection between logarithms and exponents.

Restriction	Reasoning
$b > 0$ ‍	In an exponential function, the base $b$ ‍ is always defined to be positive.
$a > 0$ ‍	$\log_{b} (a) = c$ ‍ means that $b^{c} = a$ ‍. Because a positive number raised to any power is positive, meaning $b^{c} > 0$ ‍, it follows that $a > 0$ ‍.
$b \neq 1$ ‍	Suppose, for a moment, that $b$ ‍ could be $1$ ‍. Now consider the equation $\log_{1} (3) = x$ ‍. The equivalent exponential form would be $1^{x} = 3$ ‍. But this can never be true since $1$ ‍ to any power is always $1$ ‍. So, it follows that $b \neq 1$ ‍.

Special logarithms

While the base of a logarithm can have many different values, there are two bases that are used more often than others.

Specifically, most calculators have buttons for only these two types of logarithms. Let's check them out.

The common logarithm

The common logarithm is a logarithm whose base is

10

("base-

10

logarithm").

When writing these logarithms mathematically, we omit the base. It is understood to be

10

\log_{10} (x) = \log (x)

The natural logarithm

The natural logarithm is a logarithm whose base is the number

e

("base-

e

logarithm").

Instead of writing the base as

e

, we indicate the logarithm with

\ln

\log_{e} (x) = \ln (x)

This table summarizes what we need to know about these two special logarithms:

Name	Base	Regular notation	Special notation
Common logarithm	$10$ ‍	$\log_{10} (x)$ ‍	$\log (x)$ ‍
Natural logarithm	$e$ ‍	$\log_{e} (x)$ ‍	$\ln (x)$ ‍

While the notation is different, the idea behind evaluating the logarithm is exactly the same!

Why are we studying logarithms?

As you just learned, logarithms reverse exponents. For this reason, they are very helpful for solving exponential equations.

For example the result for

2^{x} = 5

can be given as a logarithm,

x = \log_{2} (5)

. You will learn how to evaluate this logarithmic expression over the following lessons.

Logarithmic expressions and functions also turn out to be very interesting by themselves, and are actually very common in the world around us. For example, many physical phenomena are measured with logarithmic scales.

What's next?

Learn about the properties of logarithms that help us rewrite logarithmic expressions, and about the change of base rule that allows us to evaluate any logarithm we want using the calculator.

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Course: Algebra 2 > Unit 8

Intro to Logarithms

What you should be familiar with before taking this lesson

What you will learn in this lesson

What is a logarithm?

Definition of a logarithm

A helpful note

Check your understanding

Evaluating logarithms

Check your understanding

Restrictions on the variables

Special logarithms

The common logarithm

The natural logarithm

Example 1

Solution 1

Example 2

Solution 2

Why are we studying logarithms?

What's next?

Want to join the conversation?