# Lesson Plans

A half-dozen decades ago (really!), when I first started out teaching, we were given a large grade book that included pages for lesson plans. For each week of the year there were two facing pages ruled into thirty two-inch squares.

We were expected to write our lesson plans in the squares. A lesson plan consisted of something like “Product and Chain rule” or “Factor perfect square trinomials” or even just “Section 4.7.” Also, you were expected to include the homework assignment. We had to have plans written for at least two weeks in advance. Principals could collect these (although they rarely did) and check up on you.

Lesson planning changed over the years. Even though no one ever checked up on us, I soon started including more in my lesson plans. The suggested structure came to include “behavioral objectives:” brief statements of what the student should be able to do once the lesson was taught. Not a bad idea. Schools started to require the teacher to write these on the board at the beginning of the class, so students would know what they were expected to learn to do. An even better idea.

As time went on and lecturing got a bad name, you were supposed to include activities (other than copying down what you wrote on the board) in your lesson. Also, a good idea.

All this came to mind when I was asked to look at a website that offered FREE lesson plans for AP Calculus AB.

The website is Calc-medic.com. There are 150 daily lesson plans closely following the AB Calculus Course and Exam Description. The lessons are free and available to AP Calculus AB teachers. All you need to do is register. (BC lessons are planned, but since all AB topics are also BC topics, the plans will help BC teachers as well.)

If nothing more, they are a good pacing guide. BUT there is a lot more.

Before you continue, I suggest you read Tips for Lesson Planning from the Calc-Medic Blog. It discusses lesson planning, and I am sure it will be helpful whether you follow their lesson or write your own.

They call their approach “Experience First, Formalize Later” (EFFL). Each of the Calc-Medic EFFL lesson plans is organized like this:

Learning Objectives: A statement of what the lesson will teach.

Success Criteria: One or more succinct first-person statements of what students should be able to do: “I can use …”, “I can determine …”, “I can reason …”, “I can distinguish ….”

Quick Lesson Plan: Each lesson consists of four segments: (1) an activity – 15 minutes, (2) debrief [the activity] – 10 minutes, (3) Important Ideas – 10 minutes, and (4) check your understanding – 20 minutes. More about this shortly.

A brief Overview of the topic.

Teaching Tips – items you should be sure to mention with hints.

Exam Insights – notes on how the topic may appear on the exams and reference to specific AP exam questions.

Student Misconceptions – a discussion of things that may confuse students or that they may overlook.

## The Activity.

Each lesson has a handout in PDF or DOCX (so you can adapt it) and an annotated Answer Key to the activity. This is the heart of the lesson plan.

The entire lesson is in the activity handout. It begins with a set of questions that will lead the students to the topic of the lesson. These are often close to AP format and have questions based on analytic, graphic, tabular and/or a written stem. Regardless of the way the question is presented, student writing is usually included.

The activity includes a box for student notes (summarized in the answer key).

Practice questions are in the “check your understanding” part of the lesson. Teachers can use the annotated answer sheet to help decide what to present to the students and help them make their notes. To help the teacher, answers are in blue, and annotations are in red.

The lessons do not include any homework assignments. Nor are the lessons linked to any textbook. This allows you to adapt them to your textbook and situation. The authors do assign homework. They explain their philosophy in “How Do We Assign Homework?” from the blog.

There is the old generic lesson plan: (1) Tell them what you’re going to tell them, (2) Tell them, and (3) Tell them what you’ve told them. This works for classes that are primarily lectures. The EFFL lessons at Calc-Medic are more of a discovery approach. The “Activity” leads students up to the new concept(s) presented. This is then firmed up in the “Debrief” and “Important Ideas” parts of the plan and practiced in “Check Your Understanding.”

There are occasional suggestions of where to find off-site information related to the lesson, such as College Board Curriculum Modules and this blog. Links to this information are not provided; it would be helpful if they were.

Tests and Quizzes are not included. However, when the lesson calls for a test or quiz there are detailed suggestions on how to write and grade the assessment. These are generic, but nonetheless useful. There are suggestions of what to include when writing the assessment (e.g., calculator problems), what question topics to include (specific to the topics assessed), grading tips, and reflections.

The last twenty lessons are a 4-week day-by-day review for the AP Exam. Some, but by no means all, of the review lessons are linked to the Calc-medic “Review Course.” Everything mentioned above is free; the full “Review Course” is available for a per student fee.

Also worth your time is the Calc-Medic Blog with posts on topics related to AP Calculus and teaching AP Calculus. There are posts on pedagogy, the AP Exams, original videos by the authors, slide decks, and discussion of individual topics in more detail. These provide helpful insights for the teacher. It would help if these were linked to and from the lesson(s) they discuss, and if they had “tags.”

Mathmedic.com, companion website. contains similar lesson plans for Algebra 1, Geometry, Algebra 2, and Precalculus. The 180 Days of Precalculus lesson plans are not aligned with the upcoming AP Precalculus course, but lessons for this course are planned in time for the 2023 – 2024 school year. BC lesson plans are also in the works.

The sites are the work of Sarah Stecher and Barb Montgomery, teachers at East Kenwood High School in Kenwood, Michigan. They are both experienced AP Calculus Teachers. They have done a fabulous job with the lessons and their Calc-Medic Website. Both new and experienced teachers will find the lesson plans and blog helpful.

I cannot recommend Calc-Medic more highly.

ALSO:Last year I reviewed an iPad app called A Little Calculus. This app demonstrates graphically all the main concepts of AB and BC Calculus. It is quite easy to use and a quick way to prepare and present good visual examples for your class. The app has recently been updated to allow you to save and recall your own examples. Several new topics have been added. If you are not familiar with it, take a look.

P.S. Hope you like the Blog’s new look!

Updated August 15, 18, 22, 2022

I got to thinking about grading the other day after seeing a question on Facebook. We’ll get to that question in a minute, but first I want to try to outline a grading scheme I used towards the end of my teaching career. It is based on how free-response questions on AP Calculus exams are graded, but the ideas are usable in any course. Here are some suggestions and examples of how to do that. There are also some suggestions for grading multiple-choice and True-False questions.

Free-response questions

The AP Calculus scoring standards are considered as a guide for awarding partial credit. Partial credit is earned for taking correct steps on the way to the solution. Points are earned, not deducted. Examples that follow will expand on these principles:

• Each step is worth 1 or 2 points. For 2-point steps, it must be possible to earn only one point.
• Students earn the point(s) for showing they are doing a good thing.
• Once earned, the point cannot be lost by some later mistake. (It’s “in the bank,” as readers say.)
• Since a mistake will affect the final answer, the student may earn later points, including the answer point, for continuing correctly. However, some mistakes are so bad that earning the rest of the points is not possible. Mistakes must not simplify the remaining work.
• The standard must allow for different methods of solution.

Example 1: Consider a typical volume problem worth four points. Students are required to write a definite integral. By the washer method the work should look like

$\pi {{\int_{a}^{b}{{\left( {f(x)} \right)}}}^{2}}-{{\left( {g\left( x \right)} \right)}^{2}}dx=$ a numerical answer.

• 1-point is earned for the constant and both limits of integration.
• 2-points are earned for the integrand. If the integrand is of the form something squared minus something else squared, they earn 1-point; if both correct quantities are squared they earn the second point. If the integrand is something squared plus something else squared this is considered a calculus (major) mistake they earn 0 points and are not eligible for the answer point. (No deduction for a missing dx)
• 1-point for the answer from their calculator. Saying that the correct answer is equal to an incorrect integral such as $\pi {{\int_{a}^{b}{{\left( {g(x)} \right)}}}^{2}}-{{\left( {f\left( x \right)} \right)}^{2}}dx=$ the correct answer is a mistake (negative = positive) and does not earn the answer point. However, the reversed integrand and the correct answer not connected by an equal sign recoups the integrand point and earns the answer point (i.e. full credit). (Subtracting in the wrong order and taking the opposite of your (negative) answer is a correct algorithm, even if inefficient and “ugly.”)

You can see how much consideration goes into setting the grading standards.

There is no reason you must use the exact AP exam standard. In your class you may want to be more specific in hopes of helping your students be more precise. You may make this question worth more points. So, you could use this standard:

• 1-point for the $\pi$.
• 2-points for the limits of integration (one each)
• 1-point for the form of the integrand (square minus square)
• 1-point for the first squared quantity
• 1-point for the second squared quantity
• 1-point for dx
• 1-point for the answer from their calculator

Example 2: An example from Algebra 1. Find the solution of $4x-\left( {x-3} \right)=x+7$. The expected solution is

$4x-x+3=x+7$

$3x+3=x+7$

$2x=4$

$x=2$

You could count this as 3-points:

• 1-point for removing parentheses
• 1-point for collecting like terms

Or you could count it as

• 1-point for knowing to remove parentheses
• 1-point for removing parentheses correctly
• 1-point for collecting the x-terms
• 1-point for collecting the constant terms
• 1-point for the answer – any arithmetic mistakes in collecting terms fails to earn the answer point

Example 3: from Algebra 1: Solve ${{x}^{2}}-8x=9$

Expected solution:

${{x}^{2}}-8x-9=0$

$\left( {x-9} \right)\left( {x+1} \right)=0$

$x=9\text{ or }x=-1$

As a 3-point standard

• 1-point for setting equal to zero
• 1-point for factoring

Or a 5-point standard

• 1-point for setting equal to zero
• 2-points, one for each correct factor
• 2-points, one for each answer.

However, whatever method you use should allow for a solution by quadratic formula, or completing the square, or even by graphing. (Unless the direction specifically read “Solve by factoring.”)

Example 4: This is the question that got me started on this post. It is from a September 20, 2018 post on the AP Calc TEACHERS – AB/BC Facebook page. The teacher’s question is at the top.

The teacher is right about being concerned with proper notation and right about requiring students to use it.

On an AP exam this limit would be a multiple-choice question (see below) and so notation does not enter in. Even on a free-response question – judging from past exams – only the answer would be required. Just because it’s an AP class, does not mean that you must do things only as they are done on the exams.

To provide for notation, this could be scored as a 3- or 4-point question:

• 1 point for knowing what algebra to use to find the limit
• 1-point for doing the algebra correctly (For a 3-point value, this could be included in the answer point, but there is a fair amount to do and it’s not straightforward, so an additional point here is reasonable.)
• 1-point for the answer (If the student does not earn both of the first two points, then the answer should agree with their work.)
• 1-point for correct use of limit notation throughout the problem.

The student in the example does not earn the last point. The solution shown earns 3 of 4 points (or 2 of 3).

But there is more here. Suppose the student did not write “lim” in the second through fifth lines of the solution; there is no reason the must since they are just doing some algebra. Also, the lines are not connected with equal signs. Then, he or she has not misused the notation and should earn full credit.

Some comments on the Facebook post were also concerned about dividing out the x’s and not mentioning that $x\ne 0$. If that is a concern for you, then another point could be included for that.

So, the idea is to be very precise about what earns a point and what does not. Seeing a “+3” instead of a “-1” next to their work encourages students. Noticing that a number of your students are not earning the same point will help you see where the class is confused. One of the reasons you give tests is to help you see where your class as a whole is missing some idea. Consider that when giving multiple-choice and True-False questions.

Multiple-choice questions – forget scan sheets.

As a teacher, you need to see the students work, so you can find their mistakes and help them do better (a/k/a formative assessment). When giving a multiple-choice question, require students to show their work and award partial credit for incorrect answers. Two- or three-points seem to work well – one-point for knowing what to do, one-point for doing it, and one-point for the answer.

Examples:

• Find where a function is increasing: one-point for knowing to examine the derivative, one-point for finding the derivative, one point for the answer.
• Find the acceleration: one-point for finding the derivative of velocity, one-point for the answer.
• Questions with statements I, II, and III: one-point for each statement identified correctly as true or false. (Think of the answer “I only” as T,F,F etc.)
• Set up the integral: one-point for limits of integration, two-points for integrand (Algebra/notation mistake loses one point, calculus mistake loses both points).

True or False questions

For many years I used a textbook (I think it was Larson and Hostetler 2nd edition – I’m showing my age) that had True-False questions for each set of exercises. I really liked them. Newer textbooks rarely have them. One exception is the new Calculus for AP by Steward and Kokoska that starts almost every exercise set with a few True-False questions.

For two-points, have students say if the statement is true or false, AND require them to explain why the statement is true or false: what theorem or idea is illustrated; what hypothesis is not met, give counterexamples, etc.

Another approach to True-False questions is to change them to Always, Sometimes, or Never True questions.

For example: Is the statement “If f ”(a) =0, the (af(a)) is a point of inflection,” sometimes, always, or never true?

Answer: Sometimes. If f(x) = x3, then (0,0) is a point of inflection, but if f(x) = x4, then (0,0) is not a point of inflection.

Another answer: Sometimes: If f ”(x) changes sign at (af(a)), then (af(a))  is a point of inflection, if f ”(x) does not change sign at (af(a)) , then it is not a point of inflection.

You can then have the class discuss and criticize each other’s answers. These become good writing questions and good preparation for the “Justify your answer” and “Explain your reasoning” questions on the AP exams. For you, they help you see what the student is thinking and, if wrong, help them correct it (a/k/a formative assessment again).

Next Friday some thoughts On Scaling – and all tests are scaled.

# The Writing Questions on the AP Exams

The goals of the AP Calculus program state that, “Students should be able to communicate mathematics and explain solutions to problems both verbally and in well written sentences.” For obvious reasons the verbal part cannot be tested on the exams; it is expected that you will do that in your class. The exams do require written answers to parts of several questions. The number of points riding on written explanations on recent exams is summarized in the table below.

 Year AB BC 2007 9 9 2008 7 8 2009 7 3 2010 7 7 2011 7 6 2012 9 7 2013 9 7 2014 6 3 2015 8 6 2016 6 6

The average is between 6 and 8 points each year with some years having 9. That’s the equivalent of a full question. So, this is something that should not be overlooked in teaching the course and in preparing for the exams. Start long before calculus; make writing part of the school’s math program.

That a written answer is expected is indicated by phrases such as:

• Why?
• Why not?
• Explain the meaning of a definite integral in the context of the problem.
• Explain the meaning of a derivative in the context of the problem.
• Explain why an approximation overestimates or underestimates the actual value

How do you answer such a question? The short answer is to determine which theorem or definition applies and then show that the given situation specifically meets (or fails to meet) the hypotheses of the theorem or definition.

Explanations should be based on what is given in the problem or what the student has computed or derived from the given, and be based on a theorem or definition. Some more specific suggestions:

• To show that a function is continuous show that the limit (or perhaps two one-sided limits) equals the value at the point. (See 2007 AB 6)
• Increasing, decreasing, local extreme values, and concavity are all justified by reference to the function’s derivative. The table below shows what is required for the justifications. The items in the second column must be given (perhaps on a graph of the derivative) or must have been established by the student’s work.
 Conclusion Establish that y is increasing y’ > 0  (above the x-axis) y is decreasing y’ < 0   (below the x-axis) y has a local minimum y’ changes  – to + (crosses x-axis below to above) or ${y}'=0\text{ and }{{y}'}'>0$ y has a local maximum y’ changes + to –  (crosses x-axis above to below) or ${y}'=0\text{ and }{{y}'}'<0$ y is concave up y’ increasing  (going up to the right) or ${{y}'}'>0$ y is concave down y’ decreasing  (going down to the right) or ${{y}'}'<0$ y has point of inflection y’ extreme value  (high or low points) or ${{y}'}'$ changes sign.
•  Local extreme values may be justified by the First Derivative Test, the Second Derivative Test, or the Candidates’ Test. In each case the hypotheses must be shown to be true either in the given or by the student’s work.
• Absolute Extreme Values may be justified by the same three tests (often the Candidates’ Test is the easiest), but here the student must consider the entire domain. This may be done (for a continuous function) by saying specifically that this is the only place where the derivative changes sign in the proper direction. (See the “quiz” below.)
• Speed is increasing on intervals where the velocity and acceleration have the same sign; decreasing where they have different signs. (2013 AB 2 d)
• To use the Mean Value Theorem state that the function is continuous and differentiable on the interval and show the computation of the slope between the endpoints of the interval. (2007 AB 3 b, 2103 AB3/BC3)
• To use the Intermediate Value Theorem state that the function is continuous and show that the values at the endpoints bracket the value in question (2007 AB 3 a)
• For L’Hôpital’s Rule state that the limit of the numerator and denominator are either both zero or both infinite. (2013 BC 5 a)
• The meaning of a derivative should include the value and (1) what it is (the rate of change of …, velocity of …, slope of …), (2) the time it obtains this value, and (3) the units. (2012 AB1/BC1)
• The meaning of a definite integral should include the value and (1) what the integral gives (amount, average value, change of position), (2) the units, and (3) what the limits of integration mean. One way of determining this is to remember the Fundamental Theorem of Calculus $\displaystyle \int_{a}^{b}{{f}'\left( x \right)dx}=f\left( b \right)-f\left( a \right)$. The integral is the difference between whatever f represents at b and what it represents at a. (2009 AB 2 c, AB 3c, 2013 AB3/BC3 c)
• To show that a theorem applies state and show that all its hypotheses are met. To show that a theorem does not apply show that at least one of the hypotheses is not true (be specific as to which one).
• Overestimates or underestimates usually depend on the concavity between the two points used in the estimates.

A few other things to keep on mind:

• Avoid pronouns. Pronouns need antecedents. “It’s increasing because it is positive on the interval” is not going to earn any points.
• Avoid ambiguous references. Phrases such as “the graph”, “the derivative”, or “the slope” are unclear. When they see “the graph” readers are taught to ask “the graph of what?” Do not make them guess. Instead say “the graph of the derivative”, “the derivative of f”, or “the slope of the derivative.”
• Answer the question. If the question is a yes or no then say “yes” or “no.” Every year students write great explanations but never clearly say whether they are justifying a “yes” or a “no.”
• Don’t write too much. Usually a sentence or two is enough. If something extra is in the explanation and it is wrong, then the credit is not earned even though the rest of the explanation is great.

As always, look at the scoring standards from past exam and see how the justifications and explanations are worded. These make good templates for common justifications. Keep in mind that there are other correct ways to write the justifications.

QUIZ

Let $f\left( x \right)={{e}^{x}}\left( x-3 \right)$ for $0\le x\le 5$.

Find the location of the minimum value of f(x). Justify your answer three different ways (without reference to each other).

Don’t tell your students the three ways – they should know that!

The minimum value occurs at x = 2. The three ways to justify this are the First Derivative Test, the Second Derivative Test and the Candidates’ Test (aka: the Closed Interval Test). Let them discuss and constructively criticize each other’s answers. As a class, compare and contrast the students’ answers.

Next Posts:

Friday March 3: Type 1 of the 10 type questions: Rate and Accumulation

Tuesday March 7: Type 2 Linear Motion

Friday March 10: Type 3: Graph Analysis

Revised from a post of March 9, 2015.

# MPAC 6 Communicating

Saving the best, or perhaps the most important, until last, MPAC 6 is the verbal part of the Rule of Four. Problems and real-life situations are “translated” from ideas or words into symbols, equations, graphs, and tables where they are examined and manipulated to find solutions. Once the solutions are found, they must be communicated along with the reasoning involved. The aspects of good mathematical communications are those listed in this MPAC.

MPAC 6: Communicating

Students can:

a. clearly present methods, reasoning, justifications, and conclusions;

b. use accurate and precise language and notation;

c. explain the meaning of expressions, notation, and results in terms of a context (including units);

d. explain the connections among concepts;

e. critically interpret and accurately report information provided by technology; and

f. analyze, evaluate, and compare the reasoning of others.

AP® Calculus AB and AP® Calculus BC Course and Exam Description Effective Fall 2016, The College Board, New York © 2016. Full text is here.

Justifying answers and explaining reasoning in words has long been required on AP calculus exams. The exams have also required students to explain the meaning of expression involving definite integrals and the value of a derivative in the context of the questions.

How/where can you make sure students use these ideas in your classes.

Since to write mathematics well textbook authors do the things listed under this MPAC, but they rarely require students to write about or explain mathematics. They do not show students how to write good explanations of their work and solutions nor, do they provide exercises requiring explanations. Therefore, teachers must do it.

When you get to the end of the year and start working on old AP calculus exams for review you find many questions requiring students to communicate their methods and reasoning, the meanings of their work and results, the connections among different concepts, interpreting what their technology has shown them.

But waiting until the end of the year is way too late. This kind of work should be included in students’ mathematical work from the beginning, before Algebra 1. It can and should be done at every level. By the time they get to calculus, students should not be at all surprised at being asked to explain verbally and in writing what they are doing and why they chose to do it that way.

Find or provide opportunities for students to consider the reasoning of others (MPAC 6f) as well as explain their reasoning to each other. This can be accomplished with group projects, study groups, checking each other’s work, etc. You can also provide templates hits and tips for writing well.The Course and Exam Description  includes an entire section on “Representative Instructional Strategies” (pp. 33 – 37). Among the suggestions are various ways to have students work together and separately on improving their communication skills. The following section (pp. 37 – 38) discusses what a “quality response will include:

• a logical sequence of steps
• an argument that explains why those steps are appropriate, and
• an accurate interpretation of the solution (with units) in the context of the situation”

Provide less than perfect answers for students to critique and improve. (Hint: Use the sample student responses that are released each year along with the exams to show good and not-so-good answers and reasoning.

When AP exam questions are written the writers reference them to the LOs, EKs and MPACs. The released 2016 Practice Exam given out at summer institutes this summer is in the new format and contains very detailed solutions for both the multiple-choice and free-response questions that include these references. (This version is not available online as far as I know.)  None of the multiple-choice question, but all six free-response questions on both AB and BC exam reference MPAC 6 (although see 2014 AB 18 for an idea of how MPAC 6f may be tested).

Here are some previous posts on these subjects:

Writing on the AP Calculus Exams

The Opposite of Negative

What’s a Mean Old Average Anyway?

Others

PLEASE NOTE: I have no control over the advertising that appears on this blog. It is provided by WordPress and I would have to pay a great deal to not have advertising. I do not endorse anything advertised here. I noticed that ads for one of the presidential candidates occasionally appears; I certainly do not endorse him.

# Good Question 7 – 2009 AB 3

Another in my occasional series on Good Questions to teach from. This is the Mighty Cable Company question from the 2009 AB Calculus exam, number 3

This question presented students with a different situation than had been seen before. It is a pretty standard “in-out” question, except that what was going in and out was money. Students were told that the Mighty Cable Company sold its cable for $120 per meter. They were also told that the cost of the cable varied with its distance from the starting end of the cable. Specifically, the cost of producing a portion of the cable x meters from the end is $6\sqrt{x}$ dollars per meter. Profit was defined as the difference between the money the company received for selling the cable minus the cost of producing the cable. Students had a great deal of trouble answering this question. (The mean was 1.92 out of a possible 9 points. Fully, 36.9% of students earned no point; only 0.02% earned all 9 points.) This was probably because they had difficulty in interpreting the question and translating it into the proper mathematical terms and symbols. Since economic problems are not often seen on AP Calculus exams, students needed to be able to use the clues in the stem: • The$120 per meter is a rate. This should be deduced from the units: dollars per meter.
• The cost of producing the portion of cable x meters from one end cable is also a rate for the same reason. In economics this is called the marginal cost; the students did not need to know this term.
• The profit is an amount that is a function of x, the length of the cable.

Part (a): Students were required to find the profit from the sale of a 25-meter cable. This is an amount. As always, when asked for an amount, integrate a rate. In this case integrate the difference between the rate at which the cable sells and the cost of producing it.

$\displaystyle P(25)=\int_{0}^{25}{\left( 120-6\sqrt{x} \right)dx}=\2500$

or

$\displaystyle P(25)=120(25)-\int_{0}^{25}{6\sqrt{x}\ dx}=\2500$

Part (b): Students were asked to explain the meaning of $\displaystyle \int_{25}^{30}{6\sqrt{x}dx}$ in the context of the problem. Since the answer is probably not immediately obvious, here is the reasoning involved.

This is the integral of a rate and therefore, gives the amount (of money) needed to manufacture the cable. This can be found by a unit analysis of the integrand: $\displaystyle \frac{\text{dollars}}{\text{meter}}\cdot \text{meters}=\text{dollars}$ .

Let C be the cost of production, so $\displaystyle \frac{dC}{dx}=6\sqrt{x}$, and therefore, $\displaystyle \int_{25}^{30}{6\sqrt{x}dx}=C\left( 30 \right)-C\left( 25 \right)$ by the Fundamental Theorem of Calculus (FTC).

Therefore, $\displaystyle \int_{25}^{30}{6\sqrt{x}dx}$ is the difference in dollars between the cost of producing a cable 30 meters long, C(30), and the cost of producing a cable 25 meters long, C(25). (Another acceptable is that the integral is the cost in dollars of producing the last 5 meters of a 30 meter cable.)

Part (c): Students were asked to write an expression involving an integral that represents the profit on the sale of a cable k meters long.

Part (a) serves as a hint for this part of the question. Here the students should write the same expression as they wrote in (a) with the 25 replaced by k.

$\displaystyle P(k)=\int_{0}^{k}{\left( 120-6\sqrt{x} \right)dx}$

or

$\displaystyle P(k)=120k-\int_{0}^{k}{6\sqrt{x}\ dx}$

Part d: Students were required to find the maximum profit that can be earned by the sale of one cable and to justify their answer. Here they need to find when the rate of change of the profit (the marginal profit) changes from positive to negative.

Using the FTC to differentiate either of the answers in part (c) or by starting fresh from the given information:

$\displaystyle \frac{dP}{dx}=120-6\sqrt{x}$

$\displaystyle \frac{dP}{dx}=0$ when x = 400 and P(400)= $16,000. Justification: The maximum profit on the sale of one cable is$16,000 for a cable 400 meters long. For $00$ and for $x>400,{P} '(x)<0$ therefore, the maximum profit occurs at x = 400. (The First Derivative Test).

Once students are familiar with in-out questions, this is a good question to challenge them with. The actual calculus is not that difficult or unusual but concentrating on the translation of the unfamiliar context into symbols and calculus ideas is different. Show them how to read the hints in the problem such as the units.

Steel cable or steel wire rope as it is called also has some interesting geometry in its construction. You can find many good illustrations of this, such as the ones below, by Googling “steel wire rope.”

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# Matching Motion

Here’s a little matching quiz. In the function column there is a list of properties of functions and in the motion column are a list of terms describing the motion of a particle. The two lists are very similar. Match the terms in the function list with the corresponding terms in the Linear Motion list (some may be used more than once). The answers are below. For more on this idea see my previous post Motion Problems: Same Thing, Different Context.

Function                                               Linear Motion
1. Value of a function at x                     A. acceleration
2. First derivative                                  B. “at rest”
3. Second derivative                             C. farthest left
4. Function is increasing                       D. farthest right
5. Function is decreasing                      E. moving to the left or down
6. Absolute Maximum                          F. moving to the right or up
7. Absolute Minimum                            G. object changes direction
8. y ʹ = 0                                                H. position at time t
9. y ʹ changes sign                                I. speed
10. Increasing & concave up                J. speed is decreasing
11. Increasing & concave down           K. speed is increasing
12. Decreasing & concave up              L. velocity
13. Decreasing & concave down
14. Absolute value of velocity

Answers:  1. H,   2. L,   3. A,   4. F,   5. E,   6. D,   7. C,   8. B,   9. G,   10. K,   11. J,   12. J,   13. K,   14. I

# Writing on the AP Calculus Exams

The goals of the AP Calculus program state that, “Students should be able to communicate mathematics and explain solutions to problems both verbally and in well written sentences.” For obvious reasons the verbal part cannot be tested on the exams; it is expected that you will do that in your class. The exams do require written answers to a number of questions. The number of points riding on written explanations on recent exams is summarized in the table below.

 Year AB BC 2007 9 9 2008 7 8 2009 7 3 2010 7 7 2011 7 6 2012 9 7 2013 9 7 2014 6 3

The average is between 6 and 8 points each year with some years having 9. That’s the equivalent of a full question. So this is something that should not be overlooked in teaching the course and in preparing for the exams. Start long before calculus; make writing part of the school’s math program.

That a written answer is expected is indicated by phrases such as:

• Why?
• Why not?
• Explain the meaning of a definite integral in the context of the problem.
• Explain the meaning of a derivative in the context of the problem.
• Explain why an approximation overestimates or underestimates the actual value

How do you answer such a question? The short answer is to determine which theorem or definition applies and then show that the given situation specifically meets (or fails to meet) the hypotheses of the theorem or definition.

Explanations should be based on what is given in the problem or what the student has computed or derived from the given, and be based on a theorem or definition. Some more specific suggestions:

• To show that a function is continuous show that the limit (or perhaps two one-sided limits) equals the value at the point. (See 2007 AB 6)
• Increasing, decreasing, local extreme values, and concavity are all justified by reference to the function’s derivative. The table below shows what is required for the justifications. The items in the second column must be given (perhaps on a graph of the derivative) or must have been established by the student’s work.
 Conclusion Establish that y is increasing y’ > 0  (above the x-axis) y is decreasing y’ < 0   (below the x-axis) y has a local minimum y’ changes  – to + (crosses x-axis below to above) or ${y}'=0\text{ and }{{y}'}'>0$ y has a local maximum y’ changes + to –  (crosses x-axis above to below) or ${y}'=0\text{ and }{{y}'}'<0$ y is concave up y’ increasing  (going up to the right) or ${{y}'}'>0$ y is concave down y’ decreasing  (going down to the right) or ${{y}'}'<0$ y has point of inflection y’ extreme value  (high or low points) or ${{y}'}'$ changes sign.
•  Local extreme values may be justified by the First Derivative Test, the Second Derivative Test, or the Candidates’ Test. In each case the hypotheses must be shown to be true either in the given or by the student’s work.
• Absolute Extreme Values may be justified by the same three tests (often the Candidates’ Test is the easiest), but here the student must consider the entire domain. This may be done (for a continuous function) by saying specifically that this is the only place where the derivative changes sign in the proper direction. (See the “quiz” below.)
• Speed is increasing on intervals where the velocity and acceleration have the same sign; decreasing where they have different signs. (2013 AB 2 d)
• To use the Mean Value Theorem state that the function is continuous and differentiable on the interval and show the computation of the slope between the endpoints of the interval. (2007 AB 3 b, 2103 AB3/BC3)
• To use the Intermediate Value Theorem state that the function is continuous and show that the values at the endpoints bracket the value in question (2007 AB 3 a)
• For L’Hôpital’s Rule state that the limit of the numerator and denominator are either both zero or both infinite. (2013 BC 5 a)
• The meaning of a derivative should include the value and (1) what it is (the rate of change of …, velocity of …, slope of …), (2) the time it obtains this value, and (3) the units. (2012 AB1/BC1)
• The meaning of a definite integral should include the value and (1) what the integral gives (amount, average value, change of position), (2) the units, and (3) what the limits of integration mean. One way of determining this is to remember the Fundamental Theorem of Calculus $\displaystyle \int_{a}^{b}{{f}'\left( x \right)dx}=f\left( b \right)-f\left( a \right)$. The integral is the difference between whatever f represents at b and what it represents at a. (2009 AB 2 c, AB 3c, 2013 AB3/BC3 c)
• To show that a theorem applies state and show that all its hypotheses are met. To show that a theorem does not apply show that at least one of the hypotheses is not true (be specific as to which one).
• Overestimates or underestimates usually depend on the concavity between the two points used in the estimates.

A few other things to keep on mind:

• Avoid pronouns. Pronouns need antecedents. “It’s increasing because it is positive on the interval” is not going to earn any points.
• Avoid ambiguous references. Phrases such as “the graph”, “the derivative” , or “the slope” are unclear. When they see “the graph” readers are taught to ask “the graph of what?” Do not make them guess. Instead say “the graph of the derivative”, “the derivative of f”, or “the slope of the derivative.”
• Answer the question. If the question is a yes or no question then say “yes” or “no.” Every year students write great explanations but never say whether they are justifying a “yes” or a “no.”
• Don’t write too much. Usually a sentence or two is enough. If something extra is in the explanation and it is wrong, then the credit is not earned even though the rest of the explanation is great.

As always, look at the scoring standards from past exam and see how the justifications and explanations are worded. These make good templates for common justifications. Keep in mind that there are other correct ways to write the justifications.

QUIZ

Let $f\left( x \right)={{e}^{x}}\left( x-3 \right)$ for $0\le x\le 5$. Find the location of the minimum value of f(x). Justify your answer three different ways (without reference to each other).