Exponential Modeling and Logarithms

Lesson 16

Math

Unit 5

11th Grade

Lesson 16 of 16

Objective


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

Common Core Standards


Core Standards

  • 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.
  • 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.

Foundational Standards

  • F.LE.A.1.A
  • F.LE.A.1.B
  • F.LE.A.1.C

Criteria for Success


  1. Solve equations of the form $${a=bc^{dx}}$$.
  2. Solve problems using logarithm properties.
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Anchor Problems

25-30 minutes


Problem 1

  1. Lincoln deposits money in a Certificate of Deposit account. The balance (in dollars) in his account t years after making the deposit is given by $${{{{L(t)}}}=500(1.05)^t}$$ for $${{t\geq0}}$$. Explain, in terms of the structure of the expression for $${{{L(t)}}}$$, why Lincoln's balance can never be $${499}$$.

 

  1. Helen deposits money in a similar Certificate of Deposit account. The balance in her account is described, with units as in Lincoln's deposit, $${{{H(t)}}=600(1.04)^t}$$ for $${{t\geq0}}$$. Use the structure of the expressions for $${{{L(t)}}}$$ and $${{H(t)}}$$ to describe how the two balances compare over time.

 

  1. By what percent does the value of $${{{L(t)}}}$$ grow each year? What about $${{H(t)}}$$? Explain.

 

  1. During which year does Lincoln's balance to "catch up" with Helen's? Show your work.

Guiding Questions

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References

Illustrative Mathematics Comparing Exponentials

Comparing Exponentials, accessed on Feb. 23, 2018, 11:48 a.m., is licensed by Illustrative Mathematics under either the CC BY 4.0 or CC BY-NC-SA 4.0. For further information, contact Illustrative Mathematics.

Problem 2

A cup of hot coffee will, over time, cool down to room temperature. The principle of physics governing the process is Newton's Law of Cooling. Experiments with a covered cup of coffee show that the temperature (in degrees Fahrenheit) of the coffee can be modelled by the following equation

$${f(t)=110e^{−0.08t}+75}$$

Here the time $$t$$ is measured in minutes after the coffee was poured into the cup.

  1. Explain, using the structure of the expression $$ 110e^{−0.08t}+75$$, why the coffee temperature decreases as time elapses.
  2. What is the temperature of the coffee at the beginning of the experiment?
  3. After how many minutes is the coffee 140 degrees? After how many minutes is the coffee 100 degrees?

Guiding Questions

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References

Illustrative Mathematics Newton's Law of Cooling

Newton's Law of Cooling, accessed on Feb. 23, 2018, 11:53 a.m., is licensed by Illustrative Mathematics under either the CC BY 4.0 or CC BY-NC-SA 4.0. For further information, contact Illustrative Mathematics.

Target Task

5-10 minutes


Potassium-40 has a half-life of $${1.28\times 10^9}$$ years, which means that after that many years, half of a sample has decayed into a different isotope of carbon. If a sample began with $${115}$$ grams of potassium-40 $${30,000,000}$$ years ago, how much is left today? How long will it be until there are only $${50}$$ grams remaining?

Additional Practice


The following resources include problems and activities aligned to the objective of the lesson that can be used for additional practice or to create your own problem set.

  • The problems DDT-cay and Uranium 238 can be adapted to solve an equation of the form $${a=bc^{dx} }$$ to provide more practice on the skill in Anchor Problem #2.

Lesson Map

A7CB09C2-D12F-4F55-80DB-37298FF0A765

Topic A: Modeling with and Interpreting Exponential Functions

Topic B: Definition and Meaning of Logarithms

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