# 7-3 Logarithmic Functions Holt Algebra 2 Warm Up Warm Up Lesson Presentation Lesson Presentation...

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7-3Logarithmic FunctionsHolt Algebra 2Warm UpLesson PresentationLesson Quiz

Warm UpUse mental math to evaluate.1. 433. 1055. A power has a base of 2 and exponent of 4. Write and evaluate the power. (2)4 = 1620.000012. 4.

Write equivalent forms for exponential and logarithmic functions.

Write, evaluate, and graph logarithmic functions. Objectives

logarithmcommon logarithmlogarithmic functionVocabulary

How many times would you have to double $1 before you had $8? You could use an exponential equation to model this situation. 1(2x) = 8. You may be able to solve this equation by using mental math if you know 23 = 8. So you would have to double the dollar 3 times to have $8.

Holt Algebra 27-3Logarithmic Functions

How many times would you have to double $1 before you had $512? You could solve this problem if you could solve 2x = 8 by using an inverse operation that undoes raising a base to an exponent equation to model this situation. This operation is called finding the logarithm. A logarithm is the exponent to which a specified base is raised to obtain a given value.

You can write an exponential equation as a logarithmic equation and vice versa.

Write each exponential equation in logarithmic form.Example 1: Converting from Exponential to Logarithmic FormThe base of the exponent becomes the base of the logarithm.The exponent is the logarithm.An exponent (or log) can be negative.The log (and the exponent) can be a variable.log3243 = 5log1010,000 = 4logac =b

Exponential EquationLogarithmic Form35 = 24325 = 5104 = 10,00061 = ab = c

Write each exponential equation in logarithmic form.The base of the exponent becomes the base of the logarithm.The exponent of the logarithm.The log (and the exponent) can be a variable.Check It Out! Example 1a.b.c.log981 = 2log327 = 3logx1 = 0

Exponential EquationLogarithmic Form92= 8133 = 27x0 = 1(x 0)

Example 2: Converting from Logarithmic to Exponential FormWrite each logarithmic form in exponential equation.The base of the logarithm becomes the base of the power.The logarithm is the exponent.A logarithm can be a negative number.Any nonzero base to the zero power is 1. 11691 = 929 = 512b0 = 1

Logarithmic FormExponential Equationlog99 = 1log2512 = 9log82 = log4 = 2logb1 = 0

Write each logarithmic form in exponential equation.The base of the logarithm becomes the base of the power.The logarithm is the exponent.An logarithm can be negative.Check It Out! Example 2101 = 10122 = 144

Logarithmic FormExponential Equationlog1010 = 1log12144 = 2log 8 = 3

A logarithm is an exponent, so the rules for exponents also apply to logarithms. You may have noticed the following properties in the last example.

A logarithm with base 10 is called a common logarithm. If no base is written for a logarithm, the base is assumed to be 10. For example, log 5 = log105.You can use mental math to evaluate some logarithms.

Evaluate by using mental math.Example 3A: Evaluating Logarithms by Using Mental MathThe log is the exponent.Think: What power of 10 is 0.01?log 0.0110? = 0.01102 = 0.01log 0.01 = 2

Evaluate by using mental math.Example 3B: Evaluating Logarithms by Using Mental MathThe log is the exponent.Think: What power of 5 is 125?log5 1255? = 12553 = 125log5125 = 3

Evaluate by using mental math.Example 3C: Evaluating Logarithms by Using Mental MathThe log is the exponent.Think: What power of 5 is ?log5

Evaluate by using mental math.The log is the exponent.Think: What power of 10 is 0.01?log 0.0000110? = 0.00001105 = 0.01log 0.00001 = 5Check It Out! Example 3a

Evaluate by using mental math.The log is the exponent.Think: What power of 25 is 0.04? log250.0425? = 0.04251 = 0.04log250.04 = 1Check It Out! Example 3b

Holt Algebra 27-3Logarithmic Functions

Because logarithms are the inverses of exponents, the inverse of an exponential function, such as y = 2x, is a logarithmic function, such as y = log2x.You may notice that the domain and range of each function are switched.The domain of y = 2x is all real numbers (R), and the range is {y|y > 0}. The domain of y = log2x is {x|x > 0}, and the range is all real numbers (R).

Use the x-values {2, 1, 0, 1, 2}. Graph the function and its inverse. Describe the domain and range of the inverse function.Example 4A: Graphing Logarithmic Functionsf(x) = 1.25xGraph f(x) = 1.25x by using a table of values.

Example 4A ContinuedTo graph the inverse, f1(x) = log1.25x, by using a table of values.The domain of f1(x) is {x|x > 0}, and the range is R.

Example 4B: Graphing Logarithmic FunctionsUse the x-values {2, 1, 0, 1, 2}. Graph the function and its inverse. Describe the domain and range of the inverse function.

x21012f(x) =( ) x421

The domain of f1(x) is {x|x > 0}, and the range is R.To graph the inverse, f1(x) = log x, by using a table of values.Example 4B Continued

x421f 1(x) =log x21012

Check It Out! Example 4Use x = 2, 1, 1, 2, and 3 to graph . Then graph its inverse. Describe the domain and range of the inverse function.

x21123f(x) = x

Holt Algebra 27-3Logarithmic Functions

The domain of f1(x) is {x|x > 0}, and the range is R.Check It Out! Example 4

xf1(x) = log x21123

Holt Algebra 27-3Logarithmic Functions

The table lists the hydrogen ion concentrations for a number of food items. Find the pH of each.Example 5: Food Application

SubstanceH+ conc. (mol/L)Milk 0.00000025Tomatoes 0.0000316Lemon juice 0.0063

MilkExample 5 ContinuedThe hydrogen ion concentration is 0.00000025 moles per liter. pH = log[H+ ]pH = log(0.00000025)Substitute the known values in the function.Milk has the pH of about 6.6.

TomatoesThe hydrogen ion concentration is 0.0000316 moles per liter. pH = log[H+ ]pH = log(0.0000316)Substitute the known values in the function.Tomatoes have the pH of about 4.5.Example 5 Continued

Lemon juiceThe hydrogen ion concentration is 0.0063 moles per liter. pH = log[H+ ]pH = log(0.0063)Substitute the known values in the function.Lemon juice has the pH of about 2.2.Example 5 Continued

What is the pH of iced tea with a hydrogen ion concentration of 0.000158 moles per liter?The hydrogen ion concentration is 0.000158 moles per liter. pH = log[H+ ]pH = log(0.000158)Substitute the known values in the function.Iced tea has the pH of about 3.8.Check It Out! Example 5

Lesson Quiz: Part I1. Change 64 = 1296 to logarithmic form.log61296 = 43. log 100,0004. log6485. log3 Calculate the following using mental math.50.53

Holt Algebra 27-3Logarithmic Functions

Lesson Quiz: Part IID: {x > 0}; R: all real numbers

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