# Exponential Modeling and Logarithms

Students revisit exponential functions, including geometric sequences and series, and learn to manipulate logarithmic expressions and equations to solve problems involving exponential modeling.

## Unit Summary

Students have previously seen exponential functions in Algebra I. This unit builds off of that knowledge, revisiting exponential functions and including geometric sequences and series and continuous compounding situations. In the second part of the unit, students learn that the logarithm is the inverse of the exponent and to manipulate logarithmic expressions and equations. Finally, students apply their knowledge of logarithms to solve problems involving exponential modeling.

This unit is an excellent opportunity for students to practice mathematical modeling using exponential functions as models for situations in the world. Students will also look for and make use of structure as they manipulate logarithms and connect their knowledge of exponents to logarithms.

While this unit culminates the study of exponents and logarithms in the Common Core State Standards, it leads to essential topics in calculus. Students preparing to take a calculus course should emphasize algebraic manipulation of exponential and logarithmic expressions to rewrite them in a variety of ways, including using properties of logarithms and analyzing functions to connect their graphs to equations and contextual situations.

Pacing: 18 instructional days (16 lessons, 1 flex day, 1 assessment day)

## Assessment

This assessment accompanies Unit 5 and should be given on the suggested assessment day or after completing the unit.

## Unit Prep

### Intellectual Prep

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Internalization of Standards via the Unit Assessment

• Take unit assessment. Annotate for:
• Standards that each question aligns to
• Purpose of each question: spiral, foundational, mastery, developing
• Strategies and representations used in daily lessons
• Relationship to Essential Questions of unit
• Lesson(s) that assessment points to

Internalization of Trajectory of Unit

• Read and annotate "Unit Summary."
• Notice the progression of concepts through unit using the “Unit at a Glance.”
• Essential questions
• Connection to assessment questions.
• Answer the essential questions. (In the beginning, submit them to your instructional leader; toward the end, just bring them to the meeting).

Unit-Specific Intellectual Prep

### Essential Understandings

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• Write expressions or equations or change the form of an equation to model contexts and connect components of those expressions and equations to graphs and contexts.
• Explain that logarithmic functions are the inverse of exponential functions and have parallel properties to exponents used to rewrite expressions and solve equations. Know that the properties of logarithms can be used to change the form of equations to reveal solution paths.
• Use knowledge of exponents and logarithms to solve exponential modeling problems where a piece of information is missing.

### Vocabulary

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 Geometric sequence Geometric series Compounding/Continuous compounding Percentage Rate e (Euler's number) Base Rate Argument Principal Finite geometric series Sum of geometric series Summation notation ${\sum}$ Logarithm Natural log Common log Change of base Product property of logarithms (Logarithm law) Quotient property (Logarithm law) Power property (Logarithm law) Exponentiate Exponential growth Exponential decay

### Materials

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• Equations, tables, graphs, and contextual situations
• A calculator or other technology to graph and solve problems using exponential modeling and logarithms

# 1

F.IF.A.3

F.IF.B.5

F.LE.A.2

Identify, model, and analyze geometric sequences.

# 4

A.SSE.B.3.C

F.BF.A.1.A

Define and use $e$ in continuous compounding situations.

# 6

A.SSE.B.4

Find the sum of an infinite geometric series.

# 7

F.LE.A.4

Describe and evaluate simple numeric logarithms (Part I).

# 8

F.LE.A.4

Describe and evaluate simple numeric logarithms (Part II).

# 10

F.BF.B.4.C

F.LE.A.4

Evaluate common and natural logs using tables, graphs, and calculators.

# 11

F.LE.A.4

Understand and apply the change of base property to evaluate logarithms.

# 14

F.LE.A.4

Solve equations with logarithms.

# 15

A.SSE.A.1.B

F.LE.A.4

Use logarithms to solve exponential modeling problems (Part  I).

# 16

A.SSE.A.1.B

F.LE.A.4

Use logarithms to solve exponential modeling problems (Part II).

## Common Core Standards

Key: Major Cluster Supporting Cluster Additional Cluster

### Core Standards

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##### Building Functions
• F.BF.A.1 — Write a function that describes a relationship between two quantities Modeling is best interpreted not as a collection of isolated topics but in relation to other standards. Making mathematical models is a Standard for Mathematical Practice, and specific modeling standards appear throughout the high school standards indicated by a star symbol (★). The star symbol sometimes appears on the heading for a group of standards; in that case, it should be understood to apply to all standards in that group.

• F.BF.A.1.A — Determine an explicit expression, a recursive process, or steps for calculation from a context.

• F.BF.B.3 — Identify the effect on the graph of replacing f(x) by f(x) + k, k f(x), f(kx), and f(x + k) for specific values of k (both positive and negative); find the value of k given the graphs. Experiment with cases and illustrate an explanation of the effects on the graph using technology. Include recognizing even and odd functions from their graphs and algebraic expressions for them.

• F.BF.B.4 — Find inverse functions.

• F.BF.B.4.B — Verify by composition that one function is the inverse of another.

• F.BF.B.4.C — Read values of an inverse function from a graph or a table, given that the function has an inverse.

• F.BF.B.5 — Understand the inverse relationship between exponents and logarithms and use this relationship to solve problems involving logarithms and exponents.

##### Interpreting Functions
• F.IF.A.3 — Recognize that sequences are functions, sometimes defined recursively, whose domain is a subset of the integers. For example, the Fibonacci sequence is defined recursively by f(0) = f(1) = 1, f(n+1) = f(n) + f(n-1) for n ≥ 1.

• F.IF.B.4 — For a function that models a relationship between two quantities, interpret key features of graphs and tables in terms of the quantities, and sketch graphs showing key features given a verbal description of the relationship. Key features include: intercepts; intervals where the function is increasing, decreasing, positive, or negative; relative maximums and minimums; symmetries; end behavior; and periodicity. Modeling is best interpreted not as a collection of isolated topics but in relation to other standards. Making mathematical models is a Standard for Mathematical Practice, and specific modeling standards appear throughout the high school standards indicated by a star symbol (★). The star symbol sometimes appears on the heading for a group of standards; in that case, it should be understood to apply to all standards in that group.

• F.IF.B.5 — Relate the domain of a function to its graph and, where applicable, to the quantitative relationship it describes. For example, if the function h(n) gives the number of person-hours it takes to assemble n engines in a factory, then the positive integers would be an appropriate domain for the function. Modeling is best interpreted not as a collection of isolated topics but in relation to other standards. Making mathematical models is a Standard for Mathematical Practice, and specific modeling standards appear throughout the high school standards indicated by a star symbol (★). The star symbol sometimes appears on the heading for a group of standards; in that case, it should be understood to apply to all standards in that group.

• F.IF.C.7 — Graph functions expressed symbolically and show key features of the graph, by hand in simple cases and using technology for more complicated cases. Modeling is best interpreted not as a collection of isolated topics but in relation to other standards. Making mathematical models is a Standard for Mathematical Practice, and specific modeling standards appear throughout the high school standards indicated by a star symbol (★). The star symbol sometimes appears on the heading for a group of standards; in that case, it should be understood to apply to all standards in that group.

• F.IF.C.7.E — Graph exponential and logarithmic functions, showing intercepts and end behavior, and trigonometric functions, showing period, midline, and amplitude.

• F.IF.C.8 — Write a function defined by an expression in different but equivalent forms to reveal and explain different properties of the function.

• F.IF.C.8.B — Use the properties of exponents to interpret expressions for exponential functions. For example, identify percent rate of change in functions such as y = (1.02)t, y = (0.97)t, y = (1.01 12t, y = (1.2)t/10, and classify them as representing exponential growth or decay.

##### Linear, Quadratic, and Exponential Models
• F.LE.A.2 — Construct linear and exponential functions, including arithmetic and geometric sequences, given a graph, a description of a relationship, or two input-output pairs (include reading these from a table).

• F.LE.A.4 — For exponential models, express as a logarithm the solution to abct = d where a, c, and d are numbers and the base b is 2, 10, or e; evaluate the logarithm using technology.

• F.LE.B.5 — Interpret the parameters in a linear or exponential function in terms of a context.

##### Seeing Structure in Expressions
• A.SSE.A.1 — Interpret expressions that represent a quantity in terms of its context Modeling is best interpreted not as a collection of isolated topics but in relation to other standards. Making mathematical models is a Standard for Mathematical Practice, and specific modeling standards appear throughout the high school standards indicated by a star symbol (★). The star symbol sometimes appears on the heading for a group of standards; in that case, it should be understood to apply to all standards in that group.

• A.SSE.A.1.B — Interpret complicated expressions by viewing one or more of their parts as a single entity. For example, interpret P(1+r)n as the product of P and a factor not depending on P.

• A.SSE.B.3 — Choose and produce an equivalent form of an expression to reveal and explain properties of the quantity represented by the expression. Modeling is best interpreted not as a collection of isolated topics but in relation to other standards. Making mathematical models is a Standard for Mathematical Practice, and specific modeling standards appear throughout the high school standards indicated by a star symbol (★). The star symbol sometimes appears on the heading for a group of standards; in that case, it should be understood to apply to all standards in that group.

• A.SSE.B.3.C — Use the properties of exponents to transform expressions for exponential functions. For example the expression 1.15t can be rewritten as (1.151/12)12t  1.01212t to reveal the approximate equivalent monhly interest rate if the annual rate is 15%.

• A.SSE.B.4 — Derive the formula for the sum of a finite geometric series (when the common ratio is not 1), and use the formula to solve problems. For example, calculate mortgage payments. Modeling is best interpreted not as a collection of isolated topics but in relation to other standards. Making mathematical models is a Standard for Mathematical Practice, and specific modeling standards appear throughout the high school standards indicated by a star symbol (★). The star symbol sometimes appears on the heading for a group of standards; in that case, it should be understood to apply to all standards in that group.

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• N.RN.A.1

• N.RN.A.2

• F.LE.A.1

• F.LE.A.1.A

• F.LE.A.1.B

• F.LE.A.1.C

• A.REI.A.1

• A.SSE.A.2

### Standards for Mathematical Practice

• CCSS.MATH.PRACTICE.MP1 — Make sense of problems and persevere in solving them.

• CCSS.MATH.PRACTICE.MP2 — Reason abstractly and quantitatively.

• CCSS.MATH.PRACTICE.MP3 — Construct viable arguments and critique the reasoning of others.

• CCSS.MATH.PRACTICE.MP4 — Model with mathematics.

• CCSS.MATH.PRACTICE.MP5 — Use appropriate tools strategically.

• CCSS.MATH.PRACTICE.MP6 — Attend to precision.

• CCSS.MATH.PRACTICE.MP7 — Look for and make use of structure.

• CCSS.MATH.PRACTICE.MP8 — Look for and express regularity in repeated reasoning.