TBIL-Precal Properties of Logarithms (EL5)

Section 5.5 Properties of Logarithms (EL5)

Objectives

Use properties of logarithms to condense or expand logarithmic expressions.

Subsection 5.5.1 Activities

In this section, we will explore the properties of logarithms and learn how to manipulate them that will be helpful when we are ready to solve logarithmic equations.

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Remark 5.5.1.

Recall that

\log_{b} M = \log_{b} N

if and only if

M = N .

In addition, because exponentials and logarithms are inverses, we also know that

\log_{b} (b^{k}) = k .

In addition, according to the law of exponents, we know that:

x^{a} \cdot x^{b} = x^{a + b}

\frac{x^{a}}{x^{b}} = x^{a - b}

{(x^{a})}^{b} = x^{a \cdot b}

Consider all these as you move through the activities in this section.

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Activity 5.5.2.

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Let’s begin with the law of exponents to see if we can understand the product property of logs. According to the law of exponents, we know that

10^{x} \cdot 10^{y} = 10^{x + y} .

Start with this equation as you move through this activity.

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(a)

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Let

a = 10^{x}

and

b = 10^{y} .

How could you rewrite the left side of the equation

10^{x} \cdot 10^{y} ?

$a + b$
$a - b$
$10^{x + y}$
$a \cdot b$

Answer.

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(b)

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Recall from Section 5.3 that

\log_{b} M = \log_{b} N

if and only if

M = N .

Use this property to apply the logarithm to both sides of the rewritten equation from part a. What is that equation?

$\log_{10} (a + b) = \log_{10} (10^{x + y})$
$\log_{10} (a \cdot b) = \log_{10} (10^{x + y})$
$\log_{10} (a \cdot b) = \log_{10} (10^{a + b})$
$\log_{10} (a + b) = \log_{10} (10^{a + b})$

Answer.

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(c)

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Knowing that

\log_{b} (b^{k}) = k,

how could you simplify the right side of the equation?

$\log_{10} (a + b)$
$\log_{10} (x + y)$
$a + b$
$x + y$

Answer.

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(d)

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Recall in part a, we defined

10^{x} = a

and

10^{y} = b .

What would these look like in logarithmic form?

$\log_{10} a = x$
$\log_{x} a = 10$
$\log_{10} b = y$
$\log_{y} b = 10$

Answer.

A and C

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(e)

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Using your solutions in part d, how can we rewrite the right side of the equation?

$10^{a + b}$
$\log_{10} a - \log_{10} b$
$\log_{10} a + \log_{10} b$
$10^{x} + 10^{y}$

Answer.

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(f)

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Combining parts a and d, which equation represents

10^{x} \cdot 10^{y} = 10^{x + y}

in terms of logarithms?

$\log_{10} (a + b) = 10^{a + b}$
$\log_{10} (a \cdot b) = \log_{10} a - \log_{10} b$
$\log_{10} (a \cdot b) = \log_{10} a + \log_{10} b$
$\log_{10} (a \cdot b) = 10^{x} + 10^{y}$

Answer.

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Activity 5.5.3.

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According to the law of exponents, we know that

\frac{10^{x}}{10^{y}} = 10^{x - y} .

Start with this equation as you move through this activity.

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(a)

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Let

a = 10^{x}

and

b = 10^{y} .

How could you rewrite the left side of the equation

\frac{10^{x}}{10^{y}} ?

$a + b$
$a - b$
$10^{x + y}$
$a \cdot b$

Answer.

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(b)

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Recall from Section 5.3 that

\log_{b} M = \log_{b} N

if and only if

M = N .

Use this property to apply the logarithm to both sides of the rewritten equation from part a. What is that equation?

$\log_{10} (a - b) = \log_{10} (10^{x - y})$
$\log_{10} (\frac{a}{b}) = \log_{10} (10^{x - y})$
$\log_{10} (\frac{a}{b}) = \log_{10} (10^{a + b})$
$\log_{10} (a - b) = \log_{10} (10^{a - b})$

Answer.

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(c)

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Knowing that

\log_{b} (b^{k}) = k,

how could you simplify the right side of the equation?

$\log_{10} (a - b)$
$\log_{10} (x - y)$
$x - y$
$a - b$

Answer.

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(d)

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Recall in part a, we defined

10^{x} = a

and

10^{y} = b .

What would these look like in logarithmic form?

$\log_{10} a = x$
$\log_{x} a = 10$
$\log_{10} b = y$
$\log_{y} b = 10$

Answer.

A and C

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(e)

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Using your solutions in part d, how can we rewrite the right side of the equation?

$10^{a + b}$
$\log_{10} a - \log_{10} b$
$\log_{10} a - \log_{10} b$
$10^{x - y}$

Answer.

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(f)

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Combining parts a and d, which equation represents

\frac{10^{x}}{10^{y}} = 10^{x - y}

in terms of logarithms?

$\log_{10} (a - b) = 10^{a + b}$
$\log_{10} (a - b) = \log_{10} a - \log_{10} b$
$\log_{10} (\frac{a}{b}) = \log_{10} a - \log_{10} b$
$\log_{10} (\frac{a}{b}) = 10^{x - y}$

Answer.

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Remark 5.5.4.

In Activity 5.5.2 and Activity 5.5.3, you explored an example of two common properties of logarithms!

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Definition 5.5.5.

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The product property of logarithms states

\log_{a} (m \cdot n) = \log_{a} m + \log_{a} n .

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The quotient property of logarithms states

\log_{a} (\frac{m}{n}) = \log_{a} m - \log_{a} n .

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Notice that for each of these properties, the base has to be the same.

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Activity 5.5.6.

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There is still one more property to consider. This activity will investigate the power property. Suppose you are given

\log (x^{3}) = \log (x \cdot x \cdot x) .

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(a)

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By applying Definition 5.5.5, how could you rewrite the right-hand side of the equation?

$\log (3 x)$
$\log (x) - \log (x) - \log (x)$
$\log (x^{3})$
$\log (x) + \log (x) + \log (x)$

Answer.

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(b)

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By combining "like" terms, you can simplify the right-hand side of the equation further. What equation do you have after simplifying the right-hand side?

$\log (x^{3}) = \log (3 x)$
$\log (x^{3}) = - 3 \log (x)$
$\log (x^{3}) = \log (x^{3})$
$\log (x^{3}) = 3 \log (x)$

Answer.

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Definition 5.5.7.

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The power property of logarithms states

\log_{a} (m^{n}) = n \cdot \log_{a} (m) .

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Activity 5.5.8.

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Apply Definition 5.5.5 and Definition 5.5.7 to expand the following. (Note: When you are asked to expand logarithmic expressions, your goal is to express a single logarithmic expression into many individual parts or components.)

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(a)

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\log_{3} (\frac{6}{19})

$\log_{3} 6 + \log_{3} 19$
$\log_{3} (6 - 19)$
$\log_{3} 6 - \log_{3} 19$
$\log_{3} (6 + 19)$

Answer.

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(b)

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\log ((a \cdot b)^{2})

$2 (\log a + \log b)$
$2 \log a - \log b$
$\log a + 2 \log b$
$2 \log a + 2 \log b$

Answer.

A and D

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(c)

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\ln (\frac{x^{3}}{y})

$3 \ln x + \ln y$
$3 \ln x - \ln y$
$3 (\ln x - \ln y)$
$\ln x^{3} - \ln y$

Answer.

B. Note that D is also correct, but it is not expanded fully.

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(d)

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\log (x \cdot y \cdot z^{3})

$\log x + \log y + 3 \log z$
$\log x + \log y + \log z^{3}$
$\log x - \log y - 3 \log z$
$3 (\log x + y + z)$

Answer.

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Activity 5.5.9.

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Apply Definition 5.5.5 and Definition 5.5.7 to condense into a single logarithm.

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(a)

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6 \log_{6} a + 3 \log_{6} b

$6 (\log_{6} a) + 3 (\log_{6} b)$
$\log_{6} a^{6} + \log_{6} b^{3}$
$(\log_{6} a)^{6} + (\log_{6} b)^{3}$
$\log_{6} (a^{6} \cdot b^{3})$

Answer.

D. B is also correct, but it is not condensed fully.

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(b)

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\ln x - 4 \ln y

$\ln x - \ln y^{4}$
$\ln (\frac{x}{y^{4}})$
$\ln {(\frac{x}{y})}^{4}$
$\ln (x \cdot y^{4})$

Answer.

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(c)

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2 (\log (2 x) - \log y)

$\log (\frac{4 x^{2}}{y^{2}})$
$\log (\frac{2 x^{2}}{y^{2}})$
$2 \cdot \log (\frac{2 x}{y})$
$\log (\frac{4 x^{2}}{y})$

Answer.

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(d)

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\log 3 + 2 \log 5

$\log (3 \cdot 5^{2})$
$2 \cdot \log 75$
$\log 75$
$\log 225$

Answer.

C. Note that A is also correct, but not condensed completely.

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Remark 5.5.10.

You might have noticed that a scientific calculator has only "log" and "ln" buttons (because those are the most common bases we use), but not all logs have base 10 or e as their bases.

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Activity 5.5.11.

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Suppose you wanted to find the value of

\log_{5} 3

in your calculator but you do not know how to input a base other than

10

e

(i.e., you only have the "log" and "ln" buttons on your calculator). Let’s explore another helpful tool that can help us find the value of

\log_{5} 3 .

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(a)

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Let’s start with the general statement,

\log_{b} a = x .

How can we rewrite this logarithmic equation into an exponential equation?

Answer.

b^{x} = a

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(b)

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Now take the log of both sides of your equation and apply the power property of logarithms to bring the exponent down. What equation do you have now?

Answer.

x \cdot \log b = \log a

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(c)

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Solve for

x .

What does

x

equal?

Answer.

x = \frac{\log a}{\log b}

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(d)

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Recall that when we started, we defined

x = \log_{b} a .

Substitute

\log_{b} a

into your equation you got in part c for

x .

What is the resulting equation?

Answer.

\log_{b} a = \frac{\log a}{\log b}

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(e)

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Apply what you got in part d to find the value of

\log_{5} 3 .

What is the approximate value of

\log_{5} 3 ?

Answer.

\log_{5} 3

is approximately

0.683

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Remark 5.5.12.

Notice in Activity 5.5.11, we were able to calculate

\log_{5} 3

using logs of base

10 .

You should now be able to find the value of a logarithm of any base!

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Definition 5.5.13.

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The change of base formula is used to write a logarithm of a number with a given base as the ratio of two logarithms each with the same base that is different from the base of the original logarithm.