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| Student Learning Objectives: |
|---|
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| | 11 Theory slides |
| | 11 Exercises - Grade E - A |
| | Each lesson is meant to take 1-2 classroom sessions |
Sometimes the inverse of a mathematical operation is clearly identifiable. For example, addition and subtraction, multiplication and division, or raising to the power of n and calculating the n^\text{th} root are inverse operations.
A logarithm is the inverse function of an exponential function. The logarithm of a positive number m is written as log_b m and read as the logarithm of m with base b.
log_b m=n ⇔ b^n=m
Here, b is called the base in both the logarithm and the exponential expression. Logarithms are defined only for positive values of b and m, where b is not equal to 1. To see the implications of this definition, a particular example will be considered. log_4 16=n In this equation, the definition of logarithm implies that n is the exponent to which the base 4 must be raised in order to obtain 16. log_4 16= n & ⇔ 4^n=16 The following diagram illustrates how a logarithmic form has an equivalent exponential form using example numerical values.
As a consequence of the definition of a logarithm, two properties can be deduced. In these properties, b is positive and not equal to 1.
| Property | Reason |
|---|---|
| log_b b=1 | A number raised to the power of 1 is equal to itself. |
| log_b 1=0 | A number raised to the power of 0 is equal to 1. |
Paulina has recently become excited learning about logarithms.
She eagerly went to her math teacher and asked for some introductory exercises to practice evaluating and rewriting logarithmic expressions. Help her get off to a good start!
Evaluate the expression log_5 125.
Rewrite the logarithmic equation log_4 a=x as an exponential equation.
Rewrite the exponential equation 3^x=a as a logarithmic equation.
To what power must the base of 5 be raised to obtain 125?
Use the definition of a logarithm.
How can an exponential equation be rewritten using a logarithm?
To evaluate the given expression, it is helpful to recall what the definition of a logarithm is.
log_b a= c ⇔ b^c= a With this definition in mind, let x be the value of log_5 125. log_5 125= x ⇔ 5^x= 125 Given that 5 is the base, Paulina should ask herself what number it must be raised to in order to reach 125. Well, 5 to the power of 3 is equal to 125, the value of x is 3. Therefore, log_5 125=3. log_5 125= 3 ⇔ 5^3= 125
Similar to Part A, to rewrite log_4 a=x as an exponential equation, the definition of a logarithm will be used.
log_b a= c ⇔ b^c= a Therefore, Paulina should substitute b= 4 and c= x in the above definition. log_4 a= x ⇔ 4^x= a
To rewrite 3^x=a as a logarithmic equation, the definition of a logarithm will be used one last time.
b^c= a ⇔ log_b a= c Here, b= 3 and c= x will be substituted into the definition. 3^x= a ⇔ log_3 a= x
Evaluate the logarithms.
Rewrite the logarithmic equations as exponential equations and the exponential equations as logarithmic equations.
For the proof of these properties, two identities will be used. Start by recalling the definition of a logarithm. log_b a=c ⇔ a=b^c The first equation of the definition states that c=log_b a. Therefore log_b a can be substituted for c in the second equation. Furthermore, the second equation states that a is equal to b^c. This means that b^c can be substituted for a in the first equation.
| log_b a=c ⇔ a=b^c |
|---|
| a=b^c Substitute a=b^(log_b a) |
| log_b a=c Substitute log_b b^c=c |
With this information in mind, three properties can be stated.
The logarithm of a product can be written as the sum of the individual logarithms of each factor.
log_b mn=log_b m+log_b n
This property is only valid for positive values of b, m, and n, and for b≠ 1. As an example, the expression log_3 (7*4) can be rewritten using this property. log_3 (7*4)=log_3 7+log_3 4
Rewrite mn as m* n
m=b^(log_b(m))
a^m*a^n=a^(m+n)
log_b(b^m)=m
The logarithm of a quotient can be written as the difference between the logarithm of the numerator and the logarithm of the denominator.
log_b m/n=log_b m -log_b n
This property is valid for positive values of b, m, and n, and for b≠ 1. For example, the expression log_3 74 can be rewritten using this property. log_3 7/4=log_3 7-log_3 4
m=b^(log_b(m))
a^m/a^n= a^(m-n)
log_b(b^m)=m
The logarithm of a power can be written as the product of the exponent and the logarithm of the base.
log_b m^n =nlog_b m
This property is valid for positive values of b, m, and n, and for b≠ 1. For example, log_2 7^4 can be rewritten using this property. log_2 7^4=4 log_2 7
m=b^(log_b(m))
(a^m)^n=a^(m* n)
log_b(b^m)=m
Commutative Property of Multiplication
After understanding the definition of a logarithm and learning about its properties, Paulina is ready to delve deeper into this topic.
log_2 15
log_2 1/5
log_2 24
Use the Product Property of Logarithms.
Use the Quotient Property of Logarithms.
Use the Power Property of Logarithms.
Consider the Product Property of Logarithms.
log_b mn = log_b m+log_b n Knowing that log_2 3≈ 1.585 and that log_2 5≈ 2.322, this property can be used to evaluate the given expression. Start by rewriting 15 as the product of 3 and 5.
Split into factors
log_2(mn)=log_2(m) + log_2(n)
log_2 3 ≈ 1.585, log_2 5 ≈ 2.322
Add terms
Because of the quotient inside the logarithm, the Quotient Property of Logarithms will be used this time.
log_b m/n = log_b m-log_b n Also, as a consequence of the definition of a logarithm, it is known that log_b 1=0 for any positive number b different than 1. Using this information, the above property, and the fact that log_2 5≈ 2.322, the given expression can be evaluated.
log_2(a/b)=log_2(a) - log_2(b)
log_2(1) = 0
log_2 5 ≈ 2.322
Subtract term
Recall the Power Property of Logarithms and the Product Property of Logarithms.
Power:& log_b m^n = nlog_b m Product:& log_b mn = log_b m + log_b n Also, as a consequence of the definition of a logarithm, it is known that log_b b=1 for any positive number b different than 1. Using this information, the above properties, and the fact that log_2 3≈ 1.585, the given expression can be evaluated.
Split into factors
log_2(mn)=log_2(m) + log_2(n)
Write as a power
log_2(a^m)= m* log_2(a)
log_2(2) = 1
Identity Property of Multiplication
log_2 3 ≈ 1.585
Add terms
Paulina has moved beyond only understanding the definition of a logarithm. She can now rewrite exponential equations as logarithmic equations and convert logarithmic equations into exponential equations. On top of all of that, she can even evaluate logarithmic expressions. Paulina is starting to master this topic!
log_5 25x^3/y^2
log_4 sqrt(xy/4)
Start by using the Quotient Property of Logarithms. Then, use the Product Property of Logarithms. Finally, use the Power Property of Logarithms.
Start by rewriting the square root as a rational exponent. Then, use the Power Property of Logarithms to remove the exponent.
Consider the Properties of Logarithms.
Product:& log_b mn=log_b m+log_b n Quotient:& log_b m/n=log_b m-log_b n Power:& log_b m^n=nlog_b m Here, b, m, and n are positive, where b≠ 1. The main operation in the given logarithmic expression is a division. Therefore, the first property that should be used is the Quotient Property of Logarithms. Then, the Product Property of Logarithms and the Power Property of Logarithms will be applied.
log_5(a/b)=log_5(a) - log_5(b)
log_5(mn)=log_5(m) + log_5(n)
log_5(a^m)= m* log_5(a)
Calculate logarithm
To use the Power Property of Logarithms, the square root will be written as a rational exponent.
log_4 sqrt(xy/4) = log_4 (xy/4)^(12) Now, the mentioned property can be used. Then, the Quotient Property of Logarithms and the Product Property of Logarithms can also be used.
log_4(a^m)= m* log_4(a)
log_4(a/b)=log_4(a) - log_4(b)
log_4(mn)=log_4(m) + log_4(n)
Distribute 1/2
log_4(4) = 1
Identity Property of Multiplication
Paulina feels extremely confident about her skills in using logarithms! Now, instead of expanding algebraic and numeric expressions that involve logarithms, she will practice condensing them.
log_2 12 +3log_2 5-log_2 6
5log_3 2-6log_3 x+2log_3 y
Use the Product Property of Logarithms, the Power Property of Logarithms, and the Quotient Property of Logarithms.
Use the Properties of Logarithms.
To condense this expression, three properties of logarithms will be recalled first. These are the Product Property of Logarithms, the Quotient Property of Logarithms, and the Power Property of Logarithms.
Product:& log_b mn=log_b m+log_b n Quotient:& log_b m/n=log_b m-log_b n Power:& log_b m^n=nlog_b m These three properties will be used to condense the expression.
m* log_2(a)=log_2(a^m)
Calculate power
log_2(m) + log_2(n)=log_2(mn)
Multiply
log_2(m) - log_2(n)=log_2(m/n)
Calculate quotient
Similar to Part A, to condense this expression, the same three Properties of Logarithms will be applied.
m* log_3(a)=log_3(a^m)
Calculate power
log_3(m) - log_3(n)=log_3(m/n)
log_3(m) + log_3(n)=log_3(mn)
a/c* b = a* b/c
In this lesson, it has been presented that exponents and logarithms are their own inverses. This means that these two operations essentially undo each other.
| Definition | log_b a=c ⇔ a=b^c |
|---|---|
| Identity Derived From the Definition | log_b b=1 |
| Identity Derived From the Definition | log_b 1=0 |
| Product Property of Logarithms | log_b mn = log_b m+log_b n |
| Quotient Property of Logarithms | log_b m/n = log_b m-log_b n |
| Power Property of Logarithms | log_b m^n = nlog_b m |
It is important to keep in mind that these properties are only valid for positive values of a, b, m, and n, where b≠ 1. Furthermore, these properties can be used in several situations.
Order the logarithms from least to greatest value. log_5 23, log_6 38, log_7 8, log_2 10
To order the given logarithms from least to greatest value, we will find lower and upper boundaries for the numerical value of each expression. Let's start with log_5 23. Recall that 5^1=5 and that 5^2=25. With this information, we can find the values of log_5 5 and log_5 25.
cc
5^1= 5 & 5^2= 25
⇕ & ⇕
log_5 5 = 1 & log_5 25= 2
Because 23 is between 5 and 25, we can state that log_5 23 is between log_5 5 and log_5 25. This means that log_5 23 is greater than 1 and less than 2.
1
| Inequality | Comment |
|---|---|
| 1 | log_5 23 is closer to 2 than to 1 |
| 2 | - |
| 1 | log_7 8 is closer to 1 than to 2 |
| log_2 10>3 | - |
By using the information from the table, we can write the logarithmic expressions in order from least to greatest value. log_7 8, log_5 23, log_6 38, log_2 10
Let's start by writing 125 and 25, which are the base and the argument of the given logarithm, respectively, as powers with base 5. log_(125) 25 = log_(5^3) 5^2 Therefore, we need to find a number c such that 5^3 raised to the power of c equals 5^2. This can be algebraically expressed by using the definition of a logarithm. log_(5^3) 5^2= c ⇔ ( 5^3)^c= 5^2 We can use the Power of a Power Property to solve the exponential equation. Then, since both sides have the same base, we can use the Property of Equality for Exponential Equations and equate the exponents. Let's do it!
We found that the numerical value of log_(125) 25 is 23.
Similar to Part A, we can start by expressing the base of the logarithm 8 and the argument 32 as powers with base 2.
log_8 32 = log_(2^3) 2^5
We need to find a number c such that 2^3 raised to the power of c equals 2^5. Let's use the definition of a logarithm to algebraically express this.
log_(2^3) 2^5= c ⇔ ( 2^3)^c= 2^5
We can solve the exponential equation by using the Power of a Power Property and the Property of Equality for Exponential Equations.
Therefore, the numerical value of log_8 32 is 53.
In a similar manner as before, we can start by expressing the base of the logarithm 27 and the argument 81 as powers with base 3.
log_(27) 81 = log_(3^3) 3^4
We need to find a number c such that 3^3 raised to the power of c equals 3^4. Let's use the definition of a logarithm to algebraically express this.
log_(3^3) 3^4= c ⇔ ( 3^3)^c= 3^4
We can solve the exponential equation by using the Power of a Power Property and the Property of Equality for Exponential Equations.
Therefore, the numerical value of log_(27) 81 is 43.
Finally, we can express 4 and 128, which are the base and the argument of the given logarithm, as powers with base 2.
log_4 128 = log_(2^2) 2^7
We need to find a number c such that 2^2 raised to the power of c equals 2^7. Let's use the definition of a logarithm to algebraically express this.
log_(2^2) 2^7= c ⇔ ( 2^2)^c= 2^7
We can solve the exponential equation by using the Power of a Power Property and the Property of Equality for Exponential Equations.
We found that the numerical value of log_4 128 is 72.
Consider the following logarithmic expression. log x =5 What is the value of 1x?
Before finding the value of 1x, it would be convenient for us to find the value of x. To do so, we will use the definition of a logarithm. Note that we have a common logarithm, which means that the base is 10. Definition:& log_b a= c ⇔ b^c= a Given Logarithm:& log x = 5 ⇔ 10^5= x We found that x is equal to 10^5, or 100 000. With this information, we can find the value of its reciprocal 1x. x=100 000 ⇔ 1/x=1/100 000 Therefore, 1x is equal to 1100 000 or 0.00001.