<meta http-equiv="refresh" content="1; url=/nojavascript/"> Properties of Logarithms | CK-12 Foundation
Dismiss
Skip Navigation

3.4: Properties of Logarithms

Created by: CK-12
 0  0  0

Learning objectives

  • Use properties of logarithms to write logarithmic expressions in different forms.
  • Evaluate common logarithms and natural logarithms.
  • Use the change of base formula and a scientific calculator to find the values of logs with any bases.

Introduction

In the previous lesson we defined the logarithmic function as the inverse of an exponential function, and we evaluated log expressions in order to identify values of these functions. In this lesson we will work with more complicated log expressions. We will develop properties of logs that we can use to write a log expression as the sum or difference of several expressions, or to write several expressions as a single log expression. We will also work with logs with base 10 and base e, which are the bases most often used in applications of logarithmic functions.

Properties of Logarithms

Because a logarithm is an exponent, the properties of logs are the same as the properties of exponents. Here we will prove several important properties of logarithms.

Property 1: logb(xy) = logb x + logb y

Proof: Let logb x = n and logb y = m.

Rewrite both log expressions in exponential form:
logb x = nbn = x
logb y = mbm = y
Now multiply x and y: xy = bn × bm = bn + m
Therefore we have an exponential statement: bn + m = xy.
The log form of the statement is: logb xy = n + m.
Now recall how we defined n and m:
logb xy = n + m = logb x + logb y.

Property 2: log_b \left (\frac{x} {y}\right ) = log_b x - log_b y

We can prove property 2 analogously to the way we proved property 1.

Proof: Let logb x = n and logb y = m.

Rewrite both log expressions in exponential form:
logb x = nbn = x
logb y = mbm = y
Now divide x by y: \frac{x} {y} = \frac{b^n} {b^m} = b^{n - m}
Therefore we have an exponential statement: b^{n - m} = \frac{x} {y}.
The log form of the statement is: log_b \left (\frac{x} {y}\right ) = n - m.
Now recall how we defined n and m:
log_b \left (\frac{x} {y}\right ) = n - m = log_b x - log_b y.

Property 3: logb xn = n logb x

The proof of the third property relies on another property of logs that we can derive by thinking about the definition of a log. Consider the expression log2 213. What does this expression mean?

The exponential form of log2 213 = ? is 2? = 213. Looking at the exponential form should convince you that the missing exponent is 13. That is, log2 213 = 13. In general, logb bn = n. This property will be used in the proof of property 3.

Proof (of Property 3):

Let logb x = w.
The exponential form of this log statement is bw = x.
If we raise both sides of this equation to the power of n, we have (bw)n = xn.
Using the power property of exponents, this equation simplifies to bwn = xn
If two expressions are equal, then the logs of both expressions are equal:
logb bwn = logb xn
Now consider the value of the left side of the equation: logb bwn = wn.
Above, we showed that bwn = xn. By substitution, we have logb xn = wn.
Above, we defined w: logb x = w. By substitution, we have
logb xn = (logb x) n = nlogb x.

We can use these properties to rewrite log expressions.

Expanding expressions

Using the properties we have derived above, we can write a log expression as the sum or difference of simpler expressions. Consider the following examples:

  1. log2 8x = log2 8 + log2 x = 3 + log2x
  2. log_3 \left (\frac{x^2} {3}\right ) = log_3 x^2 - log_3 3 = 2log_3 x - 1

Using the log properties in this way is often referred to as “expanding”. In the first example, expanding the log allowed us to simplify, as log2 8 = 3. Similarly, in the second example, we simplified using the log properties, and the fact that log3 3 = 1.

Example 1: Expand each expression:

a. log_5 25x^2 y b. log_{10} \left (\frac{100x} {9b}\right )

Solution:

a. log5 25x2 y = log5 25 + log5x2 + log5 y = 2 + 2 log5x + log5 y
b.
log_{10} \left (\frac{100x} {9b}\right ) = log_{10} 100x - log_{10} 9b
= log_{10} 100 + log_{10} x - \left [log_{10} 9 + log_{10} b\right]
= 2 + log_{10} x - log_{10} 9 - log_{10} b

Just as we can write a single log expression as a sum and difference of expressions, we can also write expanded expressions as a single expression.

Condensing expressions

To condense a log expression, we will use the same properties we used to expand expressions. Consider the expression log6 8 + log6 27. Alone, each of these expressions does not have an integer value. The value of log6 8 is between 1 and 2; the value of log6 27 is also between 1 and 2. If we condense the expression, we get:

log6 8 + log6 27 = log6 (8 × 27) = log6 216 = 3

We can also condense algebraic expressions. This will be useful later for solving logarithmic equations.

Example 2: Condense each expression:

a. 2log3 x + log3 5x - log3 (x + 1) b. log2 (x2 - 4) - log2 (x + 2)

Solution:

a. 2log_3 x + log_3 5 x - log_3 (x + 1) = log_3 x_2 + log_3 5x - log_3 (x + 1)

= log_3 (x^2 (5x)) - log_3 (x + 1)
=log_3 \left (\frac{5x^3} {x + 1}\right )

b. log_2 (x^2 - 4) - log_2 (x + 2)= log_2 \left (\frac{x^2 - 4} {x + 2}\right )

= log_2 \left (\frac{(x + 2)(x - 2)} {x + 2}\right )
= log_2 (x - 2)

It is important to keep in mind that a log expression may not be defined for certain values of x. First, the argument of the log must be positive. For example, the expressions in example 2b above are not defined for x ≤ 2 (which allows us to "cancel" (x+2) without worrying about the condition x≠ -2).

Second, the argument must be defined. For example, in example 2a above, the expression \left (\frac{5x^3} {x + 1}\right ) is undefined if x = -1.

The log properties apply to logs with any real base. Next we will examine logs with base 10 and base e, which are the most common bases for logs (though only one is actually called “common”).

Common logarithms and natural logs

A common logarithm is a log with base 10. We can evaluate a common log just as we evaluate any other log. A common log is usually written without a base.

Example 3: Evaluate each log

a. log 1 b. log 10 c. log\sqrt{10}

Solution:

a. log 1 = 0, as 100 = 1.
b. log 10 = 1, as 101 = 10
c. log \sqrt{10} = \frac{1} {2} because \sqrt{10} = 10^{1/2}

As noted in lesson 3, logarithms were introduced in order to simplify calculations. After Napier introduced the logarithm, another mathematician, Henry Briggs, proposed that the base of a logarithm be standardized as 10. Just as Napier had labored to compile tables of log values (though his version of the logarithm is somewhat different from what we use today), Briggs was the first person to publish a table of common logs. This was in 1617!

Until recently, tables of common logs were included in the back of math textbooks. Publishers discontinued this practice when scientific calculators became readily available. A scientific calculator will calculate the value of a common log to 8 or 9 digits. Most calculators have a button that says LOG. For example, if you have TI graphing calculator, you can simply press LOG, and then a number, and the calculator will give you a log value up to 8 or 9 decimal places. For example, if you enter LOG(7), the calculator returns .84509804. This means that 10.84509804 ≈ 7. If we want to judge the reasonableness of this value, we need to think about powers of 10. Because 101 = 10, log(7) should be less than 1.

Example 4: For each log value, determine two integers between which the log value should lie. Then use a calculator to find the value of the log.

a. log 50 b. log 818

Solution:

a. log 50
The value of this log should be between 1 and 2, as 101 = 10, and 102 = 100.
Using a calculator, you should find that log 50 ≈ 1.698970004.
b. log 818
The value of this log should be between 2 and 3, as 102 = 100, and 103 = 1000.
Using a calculator, you should find that log 818 ≈ 2.912753304.

The calculator’s ability to produce log values is an example of the huge benefit that technology can provide. Only a few years ago, the calculations in the previous example would have each taken several minutes, while now they only take several seconds. While most people might not calculate log values in their every day lives, scientists and engineers are grateful to have such tools to make their work faster and more efficient.

Along with the LOG key on your calculator, you will find another logarithm key that says LN. This is the abbreviation for the natural log, the log with base e. Natural logs are written using “ln” instead of “log.” That is, we write the expression loge x as In x. How you evaluate a natural log depends on the argument of the log. You can evaluate some natural log expressions without a calculator. For example, ln e = 1, as e1 = e. To evaluate other natural log expressions requires a calculator. Consider for example ln 7. Recall that e ≈ 2.7. This tells us that ln 7 should be slightly less than 2, as (2.7)2 = 7.29. Using a calculator, you should find that ln 7 ≈ 1.945910149.

Example 5: Find the value of each natural log.

a. ln 100 b. ln \sqrt{e}

Solution:

a. ln 100 is between 4 and 5. You can estimate this by considering powers of 2.7, or powers of 3: 34 = 81, and 35 = 243.
Using a calculator, you should find that ln 100 ≈ 4.605171086.
b. Recall that a square root is the same as an exponent of 1/2. Therefore ln \sqrt{e} = ln (e^{1/2}) = 1/2

You may have noticed that the common log and the natural log are the only log buttons on your calculator. We can use either the common log or the natural log to find the values of logs with other bases.

Change of Base

Consider the log expression log3 35. The value of this expression is approximately 3 because 33 = 27. In order to find a more exact value of log335, we can rewrite this expression in terms of a common log or natural log. Then we can use a calculator.

Let’s consider a general log expression, logb x = y. This means that by = x. Recall that if two expressions are equal, then the logs of the expressions are equal. We can use this fact, and the power property of logs, to write logb x in terms of common logs.

b^y = x \Rightarrow log b^y = log x The logs of the expressions are equal
\Rightarrow y log b = log x Use the power property of logs
\Rightarrow y = \frac{log x} {log b} Divide both sides by log b
\Rightarrow log_b x = \frac{log x} {log b} Substitute log_b x = y

The final equation, log_b x = \frac{log x} {log b}, is called the change of base formula. Notice that the proof did not rely on the fact that the base of the log is 10. We could have used a natural log. Thus another form of the change of base formula is \mathit\log_b x = \frac{ln x} {ln b}.

Note that we could have used a log with any base, but we use the common log and the natural log so that we can use a calculator to find the value of an expression. Consider again log3 35. If we use the change of base formula, and then a calculator, we find that

log_3 35 = \frac{log35} {log3} = 3.23621727.

Example 6: Estimate the value, and then use the change of base formula to find the value of log2 17.

Solution: log2 17 is close to 4 because 24 = 16 and 25 = 32. Using the change of base formula, we have log_2 17 = \frac{log 17} {log 2}. Using a calculator, you should find that the approximate value of this expression is 4.087462841.

Lesson Summary

In this lesson we have developed and used properties of logarithms, including a formula that allows us to calculate the value of a log expression with any base. Out of context, it may seem difficult to understand the value of these kind of calculations. However, as you will see in later lessons in this chapter, we can use exponential and logarithmic functions to model a variety of phenomena.

Points to Consider

  1. Why is the common log called common? Why 10?
  2. Why would you want to estimate the value of a log before using a calculator to find its exact value?
  3. What kind of situations might be modeled with a logarithmic function?

Review Questions

  1. Expand the expression: logb 5x2
  2. Expand the expression: log3 81x5
  3. Condense the expression: log(x + 1) + log(x - 1)
  4. Condense the expression: 3ln(x) + 2ln (y) - ln(5x - 2)
  5. Evaluate the expressions:
    1. log 1000
    2. log 0.01
  6. Evaluate the expressions:
    1. ln e4
    2. ln \left (\frac{1} {e^9}\right )
  7. Use the change of base formula to find the value of log5 100.
  8. What is the difference between logb xn and (logbx)n?
  9. Condense the expression in order to simplify: 3 log 2 + log 125
  10. Is this equation true for any values of x and y? log2 (x + y) = log2 x + log2 y If so, give the values. If not, explain why not.

Vocabulary

Common logarithm
A common logarithm is a log with base 10k. The log is usually written without the base.
Natural logarithm
A natural log is a log with base e. The natural log is written as ln.
Scientific calculator
A scientific calculator is an electronic, handheld calculator that will do calculations beyond the four operations (+, -, \times, \div ), such as square roots and logarithms. Graphing calculators will do scientific operations, as well as graphing and equation solving operations.

Image Attributions

You can only attach files to None which belong to you
If you would like to associate files with this None, please make a copy first.

Reviews

Please wait...
Please wait...
Image Detail
Sizes: Medium | Original
 
CK.MAT.ENG.SE.1.Math-Analysis.3.4
ShareThis Copy and Paste

Original text