Author Archives: Lin McMullin

2019 CED Unit 1 – Limits and Continuity

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This is the first of a series of blog posts that I plan to write over the next few months, staying a little ahead of where you are so you can use anything you find useful in your planning. Look for this series every 2 – 4 weeks.

Unit 1 contains topics on Limits and Continuity. (CED – 2019 p. 36 – 50). These topics account for about 10 – 12% of questions on the AB exam and 4 – 7% of the BC questions.

Logically, limits come before continuity since limit is used to define continuity. Practically and historically, continuity comes first. Newton and Leibnitz did not have the concept of limit the way we use it today. It was in the early 1800’s that the epsilon-delta definition of limit was first given by Bolzano (whose work was overlooked) and then by Cauchy and Weierstrass. But their formulation did not use the word “limit”, rather the use was part of their definition of continuity. Only later was it pulled out as a separate concept and then returned to the definition of continuity as a previously defined term.

Students should have plenty of experience in their math courses before calculus with functions that are and are not continuous. They should know the names of the types of discontinuities – jump, removable, infinite, etc. As you go through this unit, you may want to quickly review these terms and concepts as they come up.

(As a general technique, rather than starting the year with a week or three of review – which the students need but will immediately forget again – be ready to review topics as they come up during the year as they are needed – you will have to do that anyway. See Getting Started #2)

Topics 1.1 – 1.9: Limits

Topic 1.1: Suggests an introduction to calculus to give students a hint of what’s coming. See Getting Started #3

Topic 21.: Proper notation and multiple-representations of limits.

There is an exclusion statement noting that the delta-epsilon definition of limit is not tested on the exams, but you may include it if you wish. The epsilon-delta definition is not tested probably because it is too difficult to write good questions. Specifically, (1) the relationship for a linear function is always  \delta =\frac{\varepsilon }{{\left| m \right|}}  where m is the slope and is too complicated to compute for other functions, and (2) for a multiple-choice question the smallest answer must be correct. (Why?)

Topic 1.3: One-sided limits.

Topic 1.4: Estimating limits numerically and from tables.

Topic 1.5: Algebraic properties of limits.

Topic 1.6: Simplifying expressions to find their limits. This can and should be done along with learning the other concepts and procedures in this unit.

Topic 1.7: Selecting the proper procedure for finding a limit. The first step is always to substitute the value into the limit. If this comes out to be number than that is the limit. If not, then some manipulation may be required. This can and should be done along with learning the other concepts and procedures in this unit.

Topic 1.8: The Squeeze Theorem is mainly used to determine \underset{{x\to 0}}{\mathop{{\lim }}}\,\frac{{\sin \left( x \right)}}{x}=1 which in turn is used in finding the derivative of the sin(x). (See Why Radians?) Most of the other examples seem made up just for exercises and tests. (See 2019 AB 6(d)). Thus, important, but not too important.

Topic 1.9: Connecting multiple-representations of limit. This can and should be done along with learning the other concepts and procedures in this unit. Dominance, Topic 15, may be included here as well (EK LIM-2.D.5)

Topics 1.10 – 1.16 Continuity

Topic 1.10: Here you can review the different types of discontinuities with examples and graphs.

Topic 1.11: The definition of continuity. The EK statement does not seem to use the three-hypotheses definition. However, for the limit to exist and for f(c) to exist, they must be real numbers (i.e. not infinite). This is tested often on the exams, so students should have practice with verifying that (all three parts of) the hypothesis are met and including this in their answers.

Topic 1.12: Continuity on an interval and which Elementary Functions are continuous for all real numbers.

Topic 1.13: Removable discontinuities and handing piecewise – defined functions

Topic 1.14: Vertical asymptotes and unbounded functions. Here be sure to explain the difference between limits “equal to infinity” and limits that do not exist (DNE). See Good Question 5: 1998 AB2/BC2.

Topic 1.15: Limits at infinity, or end behavior of a function. Horizontal asymptotes are the graphical manifestation of limits at infinity or negative infinity. Dominance is included here as well (EK LIM-2.D.5)

Topic 1.16: The Intermediate Value Theorem (IVT) is a major and important result of a function being continuous. This is perhaps the first Existence Theorem students encounter, so be sure to stop and explain what an existence theorem is.

The suggested number of 40 – 50 minute class periods is 22 – 23 for AB and 13 – 14 for BC. This includes time for testing etc. If time seems to be a problem you can probably combine topics 3 – 5, topics 6 -7, topics 11 – 12. Topics 6, 7, and 9 are used with all the limit work.

There are three other important limits that will be coming in later Units:

The definition of the derivative in Unit 2, topics 1 and 2

L’Hospital’s Rule in Unit 4, topic 7

The definition of the definite integral in Unit 6, topic 3.

Posts on Continuity

CONTINUITY To help understand limits it is a good idea to look at functions that are not continuous. Historically and practically, continuity should come before limits. On the other hand, the definition of continuity requires knowing about limits. So, I list continuity first. The modern definition of limit was part of Weierstrass’ definition of continuity.

Continuity (8-13-2012)

Continuity (8-21-2013) The definition of continuity.

Continuous Fun (10-13-2015) A fuller discussion of continuity and its definition

Right Answer – Wrong Question (9-4-2013) Is a function continuous even if it has a vertical asymptote?

Asymptotes (8-15-2012) The graphical manifestation of certain limits

Fun with Continuity (8-17-2012) the Diriclet function

Far Out! (10-31-2012) When the graph and dominance “disagree” From the Good Question series

Posts on Limits

Why Limits? (8-1-2012)

Deltas and Epsilons (8-3-2012) Why this topic is not tested on the AP Calculus Exams.

Finding Limits (8-4-2012) How to…

Limit of Composite Functions

Dominance (8-8-2012) See limits at infinity

Determining the Indeterminate (12-6-2015) Investigating an indeterminate form from a differential equation. From the Good Question series.

Locally Linear L’Hôpital (5-31-2013) Demonstrating L’Hôpital’s Rule (a/k/a L’Hospital’s Rule)

L’Hôpital’s Rules the Graph (6-5-2013)

Here are links to the full list of posts discussing the ten units in the 2019 Course and Exam Description.

2019 CED – Unit 1: Limits and Continuity

2019 CED – Unit 2: Differentiation: Definition and Fundamental Properties.

2019 CED – Unit 3: Differentiation: Composite , Implicit, and Inverse Functions

2019 CED – Unit 4 Contextual Applications of the Derivative  Consider teaching Unit 5 before Unit 4

2019 – CED Unit 5 Analytical Applications of Differentiation  Consider teaching Unit 5 before Unit 4

2019 – CED Unit 6 Integration and Accumulation of Change

2019 – CED Unit 7 Differential Equations  Consider teaching after Unit 8

2019 – CED Unit 8 Applications of Integration   Consider teaching after Unit 6, before Unit 7

2019 – CED Unit 9 Parametric Equations, Polar Coordinates, and Vector-Values Functions 

2019 CED Unit 10 Infinite Sequences and Series

Getting Started

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As you get ready to start school, here are some thoughts on the first week in AP Calculus. I looked back recently at some of the “first week of school” advice I offered in the past. Here’s a quick (actually, a bit longer than I planned) summary with some new ideas.

  1. The last time I taught AP Calculus during review time a student asked if there was a list of what’s on the exam. Duh! Why didn’t I think of that? So, I made copies of the list (from the old Acorn Book) and gave it to everyone. I should have done that on Day 1. So, my first suggestion is to make a copy of the “Mathematical Practices” and the “Course at a Glance” from the 2020 AP Calculus Course and Exam Description (p. 14 and p. 20 – 23) and give them to your students. Check off the topics as you do them during the year. A “Course at a Glance” poster comes with the hard copy of the 2019 Course and Exam Description available from the College Board or at APSIs and Workshops. See here. Hang the poster in your classroom. Refer to it often throughout the year.
  1. DON’T REVIEW! Yes, students have forgotten everything they ever learned in mathematics, but if you reteach it now, they will forget it again by the time they need it next week or next January. So, don’t waste the time, rather, plan to review material from kindergarten through pre-calculus when the topics come up during the year. Include short reviews in your lesson plans. For instance, when you study limits, you will need to simplify rational expressions – that’s when you review rational expressions. When you look at the graphs of the trigonometric functions, that’s when to review the graphs of the parent functions, a lot of the terminology related to graphs, discontinuities, asymptotes, and even the values of the trigonometric functions of the special angles. Months from now you’ll be looking at inverse functions, that’s when you review inverses.
  1. In keeping with Unit 1 Topic 1, you may want to start with a brief introduction to calculus. Several years ago, when I first started this blog, Paul A. Foerster, was nice enough to share some preview problems. They give a taste of derivatives and integrals in the first week of school and get the kids into calculus right off the bat. Here is an updated version. Paul, who retired a few years ago after 50 (!) years of teaching, is Teacher Emeritus of Mathematics of Alamo High Heights School in San Antonio, Texas. He is the author of several textbooks including Calculus: Concepts and Applications. More information about the text and accompanying explorations can be found on the first page of the explorations. Thank you, Paul!
  1. If you are not already a member, I suggest you join the AP Calculus Community. We have over 18,000 members all interested in AP Calculus. The community has an active bulletin board where you can ask and answer questions about the courses. Teachers and the College Board also post resources for you to use. College Board official announcements are also posted here. I am the moderator of the community and I hope to see you there!
  1.  Here are some links to places on this blog that you may find helpful:
    1. Pacing– organizing your year.
    2. Check the Resource page from this blog.
    3. Calculator information:
    4. Miscellany: These posts discuss basic ideas that I always hoped students knew about mathematics before starting calculus

Optimization – Reflections

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First, a new resource has been added to the resource page.  An index of all free-response questions from 1971 – 2018 listed by major topics. These were researched by by Kalpana Kanwar a teacher at Wisconsin Heights High School. Thank you Kalpana! They include precalculus topics that were tested on the exams before 1998. These may be good for your precalculus classes. (Remember that the course description underwent major changes in 1998 and some topics were dropped at that time. These include the “A” topics (precalculus), Newton’s Method, work, volume by cylindrical shells among others. Be careful, when assigning old questions; they’re good, but they may no longer be tested.) 

Optimizations problems are situations in which some item is to be made as large or small as possible. Often this is the minimum cost of producing something, or to maximize profit, or to make the largest area or volume with the least material.

While these problems are found in most of the textbooks, they almost never appear on the AP Calculus Exams. The reason for this is that the first step is to write the equation that models the situation. This step does not involve any “calculus.” If a student cannot do this or does it incorrectly, then there is no way to earn the calculus points that follow. On the exams, students are given an expression and asked to find its maximum or minimum value.

Nevertheless, the problems can be interesting and are useful in a practical sense. Reflection is one of my favorites: show that the angle of incidence equals the angle of reflection. In the figure below, light travels from a point A to point D on a reflecting surface CE and then to point B by the shortest total distance. Show that this implies that the angle α between AD and the normal to the surface is equal to the angle β between the normal and DB. The angle α is called the angle of incidence and the angle β is called the angle of reflection.

Using the lengths marked in the drawing,  \overline{{AC}},\overline{{PD}} and \overline{{BE}} are all perpendicular to  \overline{{CE}} the total distance is AD + DB. Therefore:

AD+DB=\sqrt{{{{a}^{2}}+{{x}^{2}}}}+\sqrt{{{{b}^{2}}+{{{\left( {CE-x} \right)}}^{2}}}}

To find the minimum distance find the derivative of AD + DB and set it equal to zero.

\displaystyle \frac{{2x}}{{2\sqrt{{{{a}^{2}}+{{x}^{2}}}}}}+\frac{{-2\left( {CE-x} \right)}}{{2\sqrt{{{{b}^{2}}+{{{\left( {CE-x} \right)}}^{2}}}}}}=0

Then

\displaystyle \frac{x}{{\sqrt{{{{a}^{2}}+{{x}^{2}}}}}}=\frac{{\left( {CE-x} \right)}}{{\sqrt{{{{b}^{2}}+{{{\left( {CE-x} \right)}}^{2}}}}}}

Now, we need to be clever:

\displaystyle \frac{x}{{\sqrt{{{{a}^{2}}+{{x}^{2}}}}}}=\cos \left( {ADC} \right)=\cos \left( \beta  \right) and

\displaystyle \frac{{\left( {CE-x} \right)}}{{\sqrt{{{{b}^{2}}+{{{\left( {CE-x} \right)}}^{2}}}}}}=\cos \left( {BDE} \right)=\cos \left( \alpha  \right)

And therefore, \displaystyle \alpha =\beta  QED.

See the illustration of this in Desmos here and see an easier way to do this problem.

The conic sections all have interesting reflection properties that are quite useful.

The Ellipse: A light ray leaving one focus of an ellipse is reflected by the ellipse through the other focus of the ellipse. The angle of incidence and the angle of reflection are between the segments to the foci and the normal to the ellipse.

The computation is done using a Computer Algebra System (CAS) and is shown below, the line-byline explanation follows:

  • The first line starts with the ellipse \frac{{{{x}^{2}}}}{{{{a}^{2}}}}+\frac{{{{y}^{2}}}}{{{{b}^{2}}}}=1 with a > b > 0. Solving for y there are two equations, the second one is for the upper half that we will use below. The other for the lower half.
  • The second line finds the derivative of y and the third line, m2 is the slope of the normal, the opposite of the reciprocal of the derivative.
  • The fourth and fifth lines are the slopes, m1 and m3, are the slopes from the point on the ellipse to the foci. The “such that” bar, |, indicates that what follows it is substituted into the expression.
  • The next two lines compute the inverse tangent of angle rotated counterclockwise between the segments to the foci and the normal. This uses the formula from analytic geometry: {{\tan }^{{-1}}}\left( {\frac{{{{m}_{2}}-{{m}_{1}}}}{{1+{{m}_{1}}{{m}_{2}}}}} \right)
  • The last line shows that the expression from the two lines above it are equal, indicated by the “true” on the right.

An illustration using Desmos is here. Ellipses are used as reflectors in medical and dental equipment so that a relatively dim light source can be concentrated at the place where the doctor or dentist is working without “blinding” everyone in the room. There are also ceilings that reflect sound from one focus to the other without anyone elsewhere in the room hearing. These are only a few of their uses.

The hyperbola: A light ray from one focus of a hyperbola is reflected as though it came from the other focus. This is true whether the reflection is from the side nearer the focus or farther from the focus (reflection from the convex or the concave side.

The computation is like the ellipse computation with only a few sign changes. I will not reproduce it here. If you want to try use \displaystyle \frac{{{{x}^{2}}}}{{{{a}^{2}}}}-\frac{{{{y}^{2}}}}{{{{b}^{2}}}}=1 and  c=\sqrt{{{{a}^{2}}+{{b}^{2}}}}.

There is a Desmos illustration here. Use the p-slider to move the point. The left side shows the reflection from the “outside” surface; the right side shows the reflection from the “inside” surface.

Hyperbolas are used in telescopes and magnifying mirrors to enlarge the image.

The Parabola: A light ray from the focus is reflected parallel to the axis of symmetry of the parabola Or you can go the other way: light traveling parallel to the axis is reflected to the focus.

If you try to prove this on use  x=a{{y}^{2}} to avoid working with an undefined slope. The focus is at  \left( {\frac{a}{4},0} \right).

There is a Desmos illustration here

Parabolic reflectors are used in various kinds of spotlight and telescopes and for radar dishes. They are also used for satellite dishes for cable TV; you may have one at home.

A Lesson on Sequences

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This blog post describes a lesson that investigates some ideas about sequences that are not stressed in the AP Calculus curriculum. The lesson could be an introduction to sequences. I think the lesson would work in an Algebra I course and is certainly suitable for a pre-calculus course. The investigation is of irrational numbers and their decimal representation. The successive decimal approximations to the square root of 2 is an example of a non-decreasing sequence that is bounded above and therefore converges.

Students do not need to know any of that as it will be developed in the lesson. Specifically, don’t even mention square roots, the square root of 2, or even irrational numbers until a student mention something of the sort.

This is not an efficient algorithm for finding square roots. There are far more efficient ways.

We begin with some preliminaries.

Preliminaries

  • There is a blank table that you can copy for students to use here.
  • There is a summary of the new terms used and completed table Sequence Notes. Do not give this out until after the lesson is completed.
  • We will be working with some rather long decimal numbers that will need to be squared. Scientific and graphing calculators usually compute with 14 digits and give their results rounded to 12 digits. Since ours will quickly get longer than that, I suggest you use WolframAlpha. This can be used with a computer online (at wolframalpha.com) or with an app available for smart phones and tablets. It is best if students have this website or app for their individual use.

Students will enter their numbers as shown below. Specifying “30 digits” will produces answers long enough for our purpose. To speed things up, students can edit the current number by changing the last digit in the entry line. When you get started you may have to show students how to do this. Students will need internet access.

  Computer                                                                                 Smart phone

The Lesson

The style of the lesson is Socratic. You, the teacher, will present the problem, explain how they are to go about it, and ask leading questions as appropriate. Some questions are suggested; be ready to ask others.  Later, you will have to explain (define) some new words, but as much as possible let the class suggest what to do. Drag things out of them, rather than telling them.

To begin – Produce some data

Explain to the class that they are going to generate and investigate two lists of numbers (technically called sequences). Each new member of the lists will be a number with one more decimal place than the preceding number.

The first list, whose members are called Ln, will be the largest number with the given number of decimal places, n, whose square is less than two. The subscript, n, stands for the number of decimal places in the number.

Ask: “What is the largest integer whose square is less than 2?” Answer 1, so, L0 = 1. Ask: What is the largest one place decimal whose square is less than 2?”  Answer L1 =1.4.

The second list, Gn, will be the smallest number whose square is greater than 2. So, G0 = 2 and G1 = 1.5. Notice that 1.42 = 1.96 < 2 and 1.52 = 2.25 > 2

Divide the class into 10 groups named Group 0, Group 1, Group 2, …, Group 9. In each round the groups will append their “name” to the preceding decimal and square the resulting number. Group 0 squares 1.40, group 1, squares 1.41, group 2 squares 1.42, etc. using WolframAlpha.

Ask which groups have squares less than two and enter the largest in Ln, the next number will be the smallest number whose square is greater than 2; enter it in Gn.

Complete the table by entering the largest number whose square is less than 2 in the Ln column and the smallest number whose square is greater than 2 in the Gn column. At each stage, each group appends their digit to the most recent Ln . Project or write the table on the board. Students may fill in their own copy. A completed table is here: Sequence Notes and definitions

Next – Lead a discussion

When the table is complete, prompt the students to examine the lists and come up with anything and everything they observe whether it seems important or not. Accept and discuss each observation and let the others say what they think about each observation. (Obviously, don’t deprecate or laugh at any answer – after all at this point, we don’t know what is and is not significant.)

There are (at least) three observations that are significant to what we will consider next. Hopefully, someone will mention them; keep questioning them until they do. They are these, although students may use other terms:

  1. Ln is non-decreasing. Students may first say Ln is increasing. Pause if they do and look at L12 and L13, and L15 and L16. Ask how they know Ln is non-decreasing (because each time we add a digit on the end, you get a bigger number).
  2. Likewise, Gn is non-increasing.
  3. For all n, Ln < Gn, and the numbers differ only in the last digit, and with the last digits differing by 1.

Direct instruction: Explain these ideas and terms (definition)

  • A sequence is a list or set of numbers in a given order.
  • A sequence is bounded above if there exists a number greater than or equal to all the terms of the sequence. The smallest upper bound of a sequence is called its least upper bound (l.u.b.)
  • A sequence is bounded below if there exists a number less than or equal to all the terms of the sequence. The largest lower bound is called the greatest lower bound (g.l.b.)

More questions: Apply these terms to the sequence Ln with questions like these:

  • Is Ln bounded above, below, or not bounded? (Bounded above)
  • Give an example of a number greater than all the terms of Ln. (Many answers: 1,000,000, 4, 2, 1.415, etc. and, in fact, any and every number in Gn)
  • What is the l.u.b. of Ln? Can you think of the smallest number that is an upper bound of this sequence? (Yes, \sqrt{2}. Don’t tell them this – drag it out of them if necessary.) Why? How do you know this?
  • Make the class convince you that for all n, \displaystyle \underset{{n\to \infty }}{\mathop{{\lim }}}\,\left\{ {{{L}_{n}}} \right\}=\sqrt{2}

Ask similar questions about Gn.

  • Is Gn bounded above, below, or not bounded? (Bounded below)
  • Give an example of a number less than all the terms of Ln. (Many answers: any negative number, zero, 1, 1.414, etc. Any and every number in Ln)
  • What is the g.l.b. of Gn? Can you think of the greatest number that is a lower bound of this sequence? (Yes, \sqrt{2}) Why? How do you know this?
  • Make the class convince you that for all n, \displaystyle \underset{{n\to \infty }}{\mathop{{\lim }}}\,\left\{ {{{G}_{n}}} \right\}=\sqrt{2}

 

Summing Up

Ask, “What’s happening with the numbers in the Ln sequence?” and “What’s happening to the numbers in the Gn sequence?”

The answer you want is that they are getting closer to \sqrt{2}, one from below, the other from above. (As always, wait for a student to suggest this and then let the others discuss it.)

Once everyone is convinced, explain how mathematicians say and write, “gets closer to”:

Mathematicians say that \sqrt{2} is the limiting value (or limit) of both sequences. They write \displaystyle \underset{{n\to \infty }}{\mathop{{\lim }}}\,\left\{ {{{L}_{n}}} \right\}=\sqrt{2} and \displaystyle \underset{{n\to \infty }}{\mathop{{\lim }}}\,\left\{ {{{G}_{n}}} \right\}=\sqrt{2}.

Explain very carefully that while n\to \infty is read, “n approaches infinity,” that infinity, \infty , is not a number. The symbol n\to \infty means that n gets larger without bound or that n gets larger than all (any, every) positive numbers.

In a more technical sense there is an infinite series \displaystyle \sum\limits_{{n=0}}^{\infty }{{{{a}_{n}}\cdot {{{10}}^{{-n}}}}} where \displaystyle {{a}_{n}} is one of the digits 0, 1, 2, 3, …, 9, but there is no formula for listing the values of \displaystyle {{a}_{n}}. However, the sequence of partial sum of this series is the sequence \displaystyle \left\{ {{{L}_{n}}} \right\} which converges to \displaystyle \sqrt{2}. Therefore, \displaystyle \sum\limits_{{n=0}}^{\infty }{{{{a}_{n}}\cdot {{{10}}^{{-n}}}=\sqrt{2}}}

\displaystyle \sqrt{2} is an Irrational number, but this same procedure may be used to find decimal approximation of roots of rational numbers as well. However, for Rational numbers, there are easier ways.

Finally, Irrational numbers are exactly those that cannot be written as repeating (or terminating) decimals. They “go on forever” with no pattern. The decimals you can calculate eventually stop and are rounded to the last digit. Even WolframAlpha and similar computers must eventually do this. Irrational numbers are the limits of sequences like the one we looked at today.

Exercises

  1. Follow the procedure above to find the sequence whose limit is \sqrt{{\frac{{16}}{{121}}}} . Find this number the usual way (simplify and use long division) and compare the results.
  2. Follow the procedure above to find the sequence whose limit is \sqrt{{0.390625}} . Find this number the usual way and compare the results.
  3. Using WolframAlpha determine if the computer is using Ln, Gn. both, or neither when it gives a value for \sqrt{2}. (Hint: enter “square root 2 to 5 digits” and change to 6, 7, and 8 digits; compare the answer with the sequences, you found.)

Answers:

  1. 0.363636…
  2. 0.625
  3. For n = 5 and 6 the numbers are from Ln, for n = 7 and 8 they are from Gn. WolframAlpha is using a different algorithm to compute the square root of 2; the numbers appear from both sequences due to the rounding of the answers. To see WolframAlpha’s algorithm type “square root algorithm” on the entry line. This method also produces a sequence of approximations a/b.

Revised July 28, 2021

Then there is this – Existence Theorems

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Existence Theorems

An existence theorem is a theorem that says, if the hypotheses are met, that something, usually a number, must exist.

For example, the Mean Value Theorem is an existence theorem: If a function f is defined on the closed interval [a, b] and differentiable on the open interval (a, b), then there exists a number c in the open interval (a, b) such that \displaystyle {f}'\left( c \right)\left( {b-a} \right)=f\left( b \right)-f\left( a \right).

The phrase “there exists” can also mean “there is” and “there is at least one.” In fact, it is a good idea when seeing an existence theorem to reword it using each of these other phrases. “There is at least one” reminds you that there may be more than one number that satisfies the condition. The mathematical symbol for these phrases is an upper-case E written backwards: \displaystyle \exists .

Textbooks, after presenting an existence theorem, usually follow-up with some exercises asking students to find the value for a given function on a given interval: “Find the value of c guaranteed by the Mean Value Theorem for the function … on the interval ….” These exercises may help students remember the formula involved but are not very useful otherwise.

The important thing about most existence theorems is that the number exists, not what the number is. To illustrate this, let’s consider the Fundamental Theorem of Calculus. After partitioning the interval [a, b] into subintervals at various values, xi, we consider the limit of the sum

\displaystyle \underset{{n\to \infty }}{\mathop{{\lim }}}\,\sum\limits_{{n=1}}^{n}{{\left( {f\left( {{{x}_{i}}} \right)-f\left( {{{x}_{{i-1}}}} \right)} \right)}}.

Write out a few terms and you will see that is a telescoping series and its limit is \displaystyle f\left( b \right)-f\left( a \right).

The expression \displaystyle {f\left( {{{x}_{i}}} \right)-f\left( {{{x}_{{i-1}}}} \right)} resembles the right side of the Mean Value Theorem above. Since all the conditions are met, the MVT tells us that in each subinterval \displaystyle [{{x}_{{i-1}}},{{x}_{i}}] there exists a number, call it ci , such that

\displaystyle {f}'\left( {{{c}_{i}}} \right)\left( {{{x}_{i}}-{{x}_{{i-1}}}} \right)=f\left( {{{x}_{i}}} \right)-f\left( {{{x}_{{i-1}}}} \right) and therefore

\displaystyle \underset{{n\to \infty }}{\mathop{{\lim }}}\,\sum\limits_{{n=1}}^{n}{{\left( {f\left( {{{x}_{i}}} \right)-f\left( {{{x}_{{i-1}}}} \right)} \right)}}=\underset{{n\to \infty }}{\mathop{{\lim }}}\,\sum\limits_{{n=1}}^{n}{{{f}'\left( {{{c}_{i}}} \right)\left( {{{x}_{i}}-{{x}_{{i-1}}}} \right)}}=f\left( b \right)-f\left( a \right)

No one is concerned what these ci are, just that there are such numbers, that they exist. (The second limit above is then defined as the definite integral so \displaystyle \underset{{n\to \infty }}{\mathop{{\lim }}}\,\sum\limits_{{n=1}}^{n}{{{f}'\left( {{{c}_{i}}} \right)\left( {{{x}_{i}}-{{x}_{{i-1}}}} \right)}}=\int_{a}^{b}{{{f}'\left( x \right)dx=}}f\left( b \right)-f\left( a \right) – The Fundamental Theorem of Calculus.)

Other important existence theorems in calculus

The Intermediate Value Theorem

If f is continuous on the interval [a, b] and M is any number between f(a) and f(b), then there exists a number c in the open interval (a, b) such that f(c) = M.

If f is continuous on an interval and f changes sign in the interval, then there must be at least one number c in the interval such that f(c) = 0

Extreme Value Theorem

If f is continuous on the closed interval [a, b], then there exists a number c in [a, b] such that \displaystyle f\left( c \right)\ge f\left( x \right) for all x in the interval. Every function continuous on a closed interval has (i.e. there exists) a maximum value in the interval.

If f is continuous on the closed interval [a, b], then there exists a number c in [a, b] such that \displaystyle f\left( c \right)\le f\left( x \right) for all x in the interval. Every function continuous on a closed interval has (i.e. there exists) a minimum value in the interval.

Critical Points

If f is differentiable on a closed interval and \displaystyle {f}'\left( x \right) changes sign in the interval, then there exists a critical point in the interval.

Rolle’s theorem

If a function f is defined on the closed interval [a, b] and differentiable on the open interval (a, b) and f(a) = f(b), then there must exist a number c in the open interval (a, b) such that \displaystyle {f}'\left( c \right)=0.

MVT – other forms

If I drive a car continuously for 150 miles in three hours, then there is a time when my speed was exactly 50 mph.

If a function f is defined on the closed interval [a, b] and differentiable on the open interval (a, b), then there is a point on the graph of f where the tangent line is parallel to the segment between the endpoints.

Taylor’s Theorem

If f is a function with derivatives through order n + 1 on an interval I containing a, then, for each x in I , there exists a number c between x and a such that

\displaystyle f\left( x \right)=\sum\limits_{k=0}^{n}{\frac{{{f}^{\left( k \right)}}\left( a \right)}{k!}{{\left( x-a \right)}^{k}}}+\frac{{{f}^{\left( n+1 \right)}}\left( c \right)}{\left( n+1 \right)!}{{\left( x-a \right)}^{n+1}}

The number \displaystyle R=\frac{{{f}^{\left( n+1 \right)}}\left( c \right)}{\left( n+1 \right)!}{{\left( x-a \right)}^{n+1}} is called the remainder. The equation above says that if you can find the correct c the function is exactly equal to Tn(x) + R. Notice the form of the remainder is the same as the other terms, except it is evaluated at the mysterious c. The trouble is we almost never can find the c without knowing the exact value of f(x), but; if we knew that, there would be no need to approximate. However, often without knowing the exact values of c, we can still approximate the value of the remainder and thereby, know how close the polynomial Tn(x) approximates the value of f(x) for values in x in the interval, i. See Error Bounds and the Lagrange error bound.

Cogito, ergo sum

And finally, we have Descartes’ famous “theorem” Cogito, ergo sum (in Latin) or the original French, Je pense, donc je suis, translated as “I think, therefore I am” proving his own existence.

AP Calculus Prerequisites

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College Board Prerequisites

Whenever I led a calculus workshop or APSI, I always spent a little time discussing the prerequisites for AP Calculus. Unfortunately, in some schools AP Calculus is a course for only the talented and little time is spent aligning the mathematics program and courses from 7th grade on so that more students will be able to take AP Calculus. But a program that includes the prerequisite for calculus will be a good program because of this. Such a program will also benefit students who do not take AP Calculus, but still need a good mathematics program for when they attend college.

Teachers in the earlier courses are usually appreciative of guidance from the AP Calculus teacher as to what should be included to prepare students for calculus. This is part of the rationale of the AP’s math Vertical Team program.

Below in blue is the entire prerequisite paragraph from the 2019 AP Calculus Course and Exam Description p. 7. I have separated the parts and commented on each.

Before studying calculus, all students should complete the equivalent of four years of secondary mathematics designed for college-bound students:

The four years is needed. Students should not be rushed.

In some respects, this is a political statement: four years means starting in 8th grade or earlier. While some of the most talented students can probably catch up by doing two years in one or three years in two, this is not the usual case. Learning math thoroughly takes four years.

Once in my district, our junior high decided to raise the standards for their “advanced” course that taught Algebra I in 8th grade. No one told us, so the next year we found only one class, instead of two, that could be ready for AP Calculus by the time they were seniors. We tried a three-years-in-two approach. It met with only limited success. Algebra I in 8th grade is required and really should be for everyone otherwise you are denying students the chance to even consider AP Calculus when they are seniors.

 courses that should prepare them with a strong foundation in reasoning with algebraic symbols and working with algebraic structures.

Using and understanding the use of mathematical notation is a must. Throughout the four years, algebra and its structure should be emphasized.  So, it’s not just 4 years of math, but four years of a good algebra-based math program. But algebra is not the only thing:

Prospective calculus students should take courses in which they study algebra, geometry, trigonometry, analytic geometry, and elementary functions.

All these courses are related and lead to a fuller understanding of high school math topics.

These functions include linear, polynomial, rational, exponential, logarithmic, trigonometric, inverse trigonometric, and piecewise-defined functions.

This is a list of the types of functions that should be included. They are the basic functions studied in the calculus. Linear and simple polynomial functions start in Algebra I and the others are added later. Piecewise-define functions also start early – the absolute value function is a piecewise-defined function.

In particular, before studying calculus, students must be familiar with the properties of functions, the composition of functions, the algebra of functions, and the graphs of functions.

The algebra of functions means learning how to add, subtract, multiply, divide, and compose functions and how doing so affects the properties and graphs of the resulting functions. The graphs of these functions and how doing algebra, composition, and transformations affects the graph is important.

Students must also understand the language of functions (domain and range, odd and even, periodic, symmetry, zeros, intercepts, and descriptors such as increasing and decreasing).

The list of the language functions is too short. Some terms such as increasing, decreasing, maximum and minimum values, concavity and others often considered the province of calculus all come up in the study of functions and can and should be discussed when they arise using the correct terminology and notation. There is no need to wait for calculus to use them to describe functions, graphs and transformations. An informal use and understanding of continuity and limits should be included. Asymptotes should not be overlooked (they are the graphical manifestation of limits and continuity or the lack of same). The more students learn before calculus, the less you’ll have to do in calculus.

Students should also know how the sine and cosine functions are defined from the unit circle and know the values of the trigonometric functions at the numbers 0,\tfrac{\pi }{6},\tfrac{\pi }{4},\tfrac{\pi }{3},\tfrac{\pi }{2} and their multiples.

Yes, with all the technology available these basic trig facts should be learned (learned, not just memorized); they are always tested on the AP Exams.

Students who take AP Calculus BC should have basic familiarity with sequences and series, as well as some exposure to parametric and polar equations.

Here I disagree. Parametric equations, vector equations and polar equations should be a part of the curriculum for all students. Students who do not take BC calculus, may well take more math courses in college and should understand these ways of working with the plane and with functions defined in different ways.

This list does not define the entire high school math program. There are other topics that can and probably should be included – statistics, systems of equations, linear algebra and matrices, proofs, probability to name a few. What it does define is what should be included so that students will be ready for calculus.

What I think is missing here is the use of technology. In the world today mathematics is done with technology. The proper use of technology should be an integral part of the program from before Algebra I.

AP Statistics is a great course. Students who have completed Algebra II should consider this course. However, AP Statistics it is not an algebra-based course. About three-quarters of the course and its exam is writing; there is very little algebra involved. Therefore, students should not be taking AP Statistics instead of AP Calculus, or if they are not taking calculus, instead of a third year of Algebra. The AP Statistic prerequisites state:

Students who wish to leave open the option of taking calculus in college should include precalculus [i.e. a third year of algebra] in their high school program and perhaps take AP Statistics concurrently with precalculus.

Students with the appropriate mathematical background are encouraged to take both AP Statistics and AP Calculus in high school.

— AP Statistics 2019 Course and Exam Description p. 7, emphasis added.

The point is that students should not have a year in high school without an algebra course. A year in which to forget their algebra before going to college where they may need it again is not a good idea.

I like to think of all the mathematics courses before calculus as “precalculus.” In many schools, “precalculus” is the name of the last course before calculus. That’s okay, I guess. What I disagree with is that often the precalculus teacher, with the good intention of preparing their students for calculus, teaches them “derivatives.” By which they mean the rules for computing derivatives. This really does not help the students or the calculus teacher.

Derivatives are limits and derivatives are slopes; computing derivatives is the least of your worries. If students have learned all the other precalculus topics (including parametric, vector, and polar equations) well and there is time left, consider delving further into limits and continuity. Limits seem to be more difficult to understand and some repeating of the topic when students arrive in calculus will do no harm. Leave the calculus for the calculus class. (The exception is when the precalculus class is intentionally meant to get an early start on the calculus; when it is taught by the calculus teacher or a teacher who is aware of the Essential Knowledge and Learning objective of the AP Calculus course.)  – Just my opinion.

High School Prerequisites

Some high schools add their own prerequisites to enter AP Calculus courses. This usually means students have to earn a significantly higher score than just a passing grade in the precalculus course(s). I do not agree with such a policy.  It excludes students who may benefit. If your student passed the precalculus course, even with a low grade, how can you say they are not ready for calculus? What will make them more ready? True, they may have to struggle, but that won’t hurt them. You may want to council them (and their parents) and explain, without discouraging them, the amount of work and time required in a college level course like AP. Explain the amount of time and work they will have to spend once they get to college in a course that meets far fewer times then AP Calculus to cover the same material. Even if they end up without earning a qualifying score on the AP Exam, they will still benefit by putting in the time and effort. If they want to try, encourage them.

Mathematical Practices

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In March, I attending a training session given by the College Board on the new 2019 AP Calculus Course and Exam Description (2019 CED). I was impressed by the copious other materials the College Board had prepared for the roll-out that will be available at summer institutes. Among these was Mathematical Practices. The MPACs (Mathematical Practices) from the 2016 CED have been revised and condensed from six down to four. In both forms they summarize how mathematicians work, think, and communicate. Therefore, they outline what students need to learn and do when learning mathematics.

The Practices are summarized on page 13 – 14 of the 2019 CED and discussed in detail in the “Developing the Mathematical Practices” chapter (p. 214 – 220) where, included with each of the skills, are Key Questions, Sample Activities, and Sample Instructional Strategies. Each unit in the 2019 CED starts with a short discussion of the Mathematical Practices that apply to that unit.

While the Practices are listed with examples specifically for the AP Calculus courses, they really apply to the entirety of a student’s mathematical learning and thinking from grade school on. If your school district has a Math Vertical Team, an ongoing discussion of the Practices is certainly an appropriate topic. Otherwise, share them with the teachers from the lower grades and sending schools. They are relevant at all grade levels.

One thing you can do to help students with the Practices is to make and keep them aware of them. Put them on a poster in the room. Make a handout of pages 13 and 14 for the front of their notebook. Refer to them whenever you use one of the items on the list.

The practices are these. (I have slightly edited them to remove the numbering and the calculus-specific examples.) My thoughts and comments are below the quotes.

Practice 1: Implementing Mathematical Processes – Determine expressions and values using mathematical procedures.

  • Identify the question to be answered or problem to be solved.
  • Identify key and relevant information to answer a question or solve a problem.
  • Identify an appropriate mathematical rule or procedure based on the classification of a given expression.
  • Identify an appropriate mathematical rule or procedure based on the relationship between concepts or processes to solve problems.
  • Apply appropriate mathematical rules or procedures, with and without technology.
  • Explain how an approximated value relates to the actual value.

The first Practice really describes the problem-solving process. This Practice is applicable throughout a student’s study of mathematics from grade school on.

The first two bullets while marked as “not assessed [on the AP Calculus exams]” are the beginning of the problem-solving process. The next two are how you start the work of problem solving, and the fifth applies to carrying out the rules and procedure you’ve decided upon. The last needs to be considered whenever your answer is not exact – which may be most of the time.

Practice 2: Connecting Representations – Translate mathematical information from a single representation or across multiple representations.

  • Identify common underlying structures in problems involving different contextual situations.
  • Identify mathematical information from graphical, numerical, analytical, and/or verbal representations.
  • Identify a re-expression of mathematical information presented in a given representation.
  • Identify how mathematical characteristics or properties of functions are related in different representations.
  • Describe the relationships among different representations of functions ….

Multiple representations, often called the “Rule of Four”, help one see and delve deeper into mathematical situations. Graphs, tables, and symbolic expressions representing the same thing show different ways of expressing and understanding mathematical ideas. Expressing the relationships in words by writing, talking, discussing, and arguing about them helps students understand and internalize the mathematics (see Practice 4). Technology is invaluable in doing this.

All four should be considered in every situation and for every concept. Sometimes one is more informative and useful than the others, other times a different perspective sheds additional light on the concept. And, once again, this should be done from the beginning of a student’s mathematical career.

Practice 3: Justification – Justify reasoning and solutions

  • Apply technology to develop claims and conjectures.
  • Identify an appropriate mathematical definition, theorem, or test to apply.
  • Confirm whether hypotheses or conditions of a selected definition, theorem, or test have been satisfied.
  • Apply an appropriate mathematical definition, theorem, or test.
  • Provide reasons or rationales for solutions and conclusions.
  • Explain the meaning of mathematical solutions in context.
  • Confirm that solutions are accurate and appropriate.

Technologies (in the broad sense of anything other than paper and pencil: blocks, beads on wires, and other manipulatives in grade school, to computer programs, spreadsheets, CAS, and Oh BTW, graphing calculators) are an increasingly important tool for mathematicians. Technology should be incorporated at all grades and levels. Students should learn how to use them no only to do and check their work, but also to explore mathematics and discover mathematical ideas (even if these are already known to more advanced students).

Definitions and theorems formalize the results of mathematical exploration and point the way to other discoveries. Students should become familiar, not just with a few theorems and definitions, but with the structure of them and relationships between them (converses, inverses, and contrapositives). They need to know that if the hypotheses are true, then the conclusion is true. They need to be able to show (confirm) that the hypotheses are true before they apply a theorem or definition to a given situation.

In early grades, stating theorem formally is not always necessary or desirable. Still, students should be aware that there are certain rules (which after all are theorems) and they may be used only when appropriate. I’ve often told students that in real life you can do whatever you want unless there is a law saying you can’t, but in mathematics you can’t do anything unless there is a law that say you can.

Part of the problem-solving process in Practice 1 should include making sure your result makes sense in context. That means student mathematicians need to understand the meaning of their results and be able to confirm that the work and the solution are accurate and appropriate. Explaining this verbally to other and in writing, a communication skill from Practice 4, is a way to do this. This can be does at all grade levels.

The previous MPACs from the 2016 CED list “Students can … analyze, evaluate, and compare the reasoning of others.” (MPAC 6f.) At all levels, this is one way to have students confirm and explain their results and understanding.

Practice 4: Communication and Notation – Use correct notation, language, and mathematical conventions to communicate results or solutions.

  • Use precise mathematical language.
  • Use appropriate units of measure.
  • Use appropriate mathematical symbols and notation
  • Use appropriate graphing techniques.
  • Apply appropriate rounding procedures.

As we’ve all learned early in our teaching careers, after teaching a topic two or three times we understand it much better. We see the fine points and appreciate the connections. It was that communication, the teaching of it, that helped us understand it. Activities where students communicate help them understand as well.

The items under Practice 4, are important because communication with others orally and in writing will help your students learn and understand mathematics. To use the language of mathematics, students need to know the structure of mathematical reasoning (return to Practice 3 – theorems and definitions), and the tools for doing so (notation, units, etc.). At all grade levels, students should practice in communicating and using the language and notation – this will help them learn.

Take a good look at the Mathematical Practices and incorporate them into your thinking and teaching. Help your students look at what they are doing, to look at the big picture. It will help with the details.