Author: John Quintanilla

I'm a Professor of Mathematics and a University Distinguished Teaching Professor at the University of North Texas. For eight years, I was co-director of Teach North Texas, UNT's program for preparing secondary teachers of mathematics and science.

Engaging students: The field axioms

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission again comes from my former student Andrew Sansom. His topic, from Pre-Algebra: the field axioms of arithmetic (the distributive law, the commutativity and associativity of addition and multiplication, etc.).

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Algebra, from one perspective, is the use of numbers’ and operations’ properties to manipulate expressions. Some of these properties, called the field axioms, are crucial to being able to easily solve equations. These properties include associativity, commutativity, distributivity, identity, and inverse. To better appreciate how these properties are so helpful in algebra, it is useful to explore some examples of operations that do not obey these laws.

A1. What interesting (i.e., uncontrived) word problems using this topic can your students do now?

Example 1: The Average (Mean) is Not Associative

Part 1
A math teacher Mrs. Taylor instructs a class of three students: Alice, Bob, and Charlie. The class took an exam last week, but Charlie was sick and missed the test, so he took it today. Mrs. Taylor promised the class that if the class average on the exam was high enough, she would give them all candy. If Alice scored a 96 and Bob scored an 83, what was the class average (the average of those two students) after the first day of the exam?

mean(A,B)= \frac{(A+B}{2}=

Part 2
After Charlie took the exam (he scored an 89), Mrs. Taylor wanted to know if she had to calculate the average from scratch (i.e. add all three scores and divide by three), or if she could just average the previous mean and Charlie’s score (i.e. add your answer from part 1 and Charlie’s score and divide by 2), since she already had done some arithmetic and didn’t want to waste time. Would she find the same answer if she tried both methods? If not, which one is correct? Why?
mean(mean(A,B),C)= \frac{ \frac{A+B}{2} +C}{2} =

mean(A,B)= \frac{A+B+C}{3}=

Part 3
After her discovery in Part 2, Mrs. Taylor is curious if she first found the mean of Bob and Charlie’s grades, then averaged it with Alice’s grade, if it would be the same as an answer above. Is it? Why or why not?

mean(A,mean(B,C))=\frac{A+ \frac{B+C}{2} }{2}=

Part 4
What does it mean for an operation to be associative? How does this activity show that the average (mean) is not associative? Why does this mean you have to be extra careful when solving problems with averages?

Example 2: Subtraction is Not Commutative

Part 1
Mrs. Taylor likes to visit Alaska during the summer. When she arrived in Anchorage, it was 10F, but a snowstorm caused the temperature to drop by 21F. Write an equation with subtraction to find the new temperature the next day.

The next summer, when Mrs. Taylor arrives in Anchorage, it is 21F but the temperature drops 10F. Write an equation with subtraction to find the new temperature the next day.

Part 2
What does it mean for an operation to be commutative? Based on what you found in Part 1, is subtraction commutative? Why or why not? Why does that mean you need to be extra careful when solving problems with subtraction?

 

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B2. How does this topic extend what your students should have learned in previous courses?

Prior to pre-algebra, students should be proficient in arithmetic. In that study, they should have been exposed to fact families, which are simple examples of the inverse elements of addition and multiplication. The field axioms generalize these ideas to other objects. Students also should have realized that subtraction and division do not commute, though they likely never used that name. They also likely realized that addition by 0 or multiplication by 1 do not affect the value of the other element. By learning the names of these different properties, students build upon their prior experience to be able to label and acknowledge when these properties appear in other contexts.

 

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B1. How can this topic be used in your students’ future courses in mathematics or science?

Although high school students will spend most of their time working in fields, instead of other algebraic structures such as non-Abelian groups or noncommutative rings, an appreciation and awareness of the field axioms while studying pre-algebra will prepare them for solving equations involving exponents (for example, intuitively questioning whether 2^x=x^2, which are trivially different, but not obvious to the novice). Furthermore, most Algebra II classes do briefly study Matrix Algebra, which is noncommutative (i.e. matrix multiplication does not commute), which causes many interesting conundrums for the uninitiated student while trying to solve problems. This appreciation of the field axioms prepares them for later study in Linear Algebra and Abstract Algebra. Outside of their math classes, vector fields form a critical part of physics, even at the high school level. Although most high school students do not realize it, they have to use the field axioms all the time to solve physics problems.

References:
Use of the mean as a simple example of a non-associative operation courtesy of StackExchange user “Accumulation” on the thread “Non-Associative Operations” (https://math.stackexchange.com/a/2892589)

 

Living Proof: Stories of Resilience Along the Mathematical Journey

Quoting shamelessly from https://blogs.ams.org/inclusionexclusion/2019/06/26/living-proof-a-must-read/:

The AMS and MAA have recently published a phenomenal collection of essays entitled “Living Proof: Stories of Resilience Along the Mathematical Journey”, edited by Allison K. Henrich, Emille D. Lawrence, Matthew A. Pons, and David G. Taylor. The book is free, and features an astounding group of contributing authors. The stories are organized around common themes in the experiences. Part I is about math getting hard and people hitting a wall. Part II is about struggling to belong in math (and is particularly well aligned with the goals of this blog). Part III is about persevering through and overcoming difficulties. And Part IV is about the sometimes challenge of integrating our mathematical identities with the rest of our lives.

I’ve read this compendium of 41 two- and three-page essays myself, and I highly recommend it as a way of encouraging young mathematician to persist along the journey.

Mathematics is about wonder, creativity and fun, so let’s teach it that way

I enjoyed this opinion piece at phys.org about project-based instruction in mathematics. A sample quote:

Mathematician Jo Boaler from the Stanford Graduate School of Education says that a “wide gulf between real mathematics and school mathematics is at the heart of the math problems we face in school education.”

Of the subject of mathematics, Boaler notes that: “Students will typically say it is a subject of calculations, procedures, or rules. But when we ask mathematicians what math is, they will say it is the study of patterns that is an aesthetic, creative, and beautiful subject. Why are these descriptions so different?”

She points out the same gulf isn’t seen if people ask students and English-literature professors what literature is about.

In the process of constructing the RabbitMath curriculum, problems or activities are included when team members find them engaging and a challenge to their intellect and imagination. Following the analogy with literature, we call the models we are working with mathematical novels.

 

Students Find Glaring Discrepancy in US News Rankings

Despite its hopelessly flawed methodology, U.S. News & World Report continues to sell magazines with its lists of Top 25 or Top 100 universities in various categories. Some universities who don’t play along, like Reed College, have long suspected that their rankings are penalized. So I enjoyed this press release from Reed College about statistics students who reverse-engineered the rankings to measure the magnitude of this penalty. The results are startling: while Reed was officially ranked #90, the formula should have them at about #38. In one glaring example, the magazine underestimated the college’s financial resources by over 100 spots even though this information the magazine could have obtained this information from free government databases instead of their survey.

To Save The Science Poster, Researchers Want To Kill It And Start Over

Professional conferences often feature poster sessions, and, more often than not, the poster is simply incomprehensible to somebody walking through the aisles.

So I enjoyed this article about an innovative way to bring scientific posters in the 21st century. The money quote:

“The current method is not effective in communicating research findings. For instance, in my field, we all want improvements in our life: vaccines for all diseases, easier delivery of vaccines, innovative way to finance vaccines, effective ways tackling vaccine hesitancy,” Suharlim says. “Experts are all coming to these conferences, and they have limited time to update their knowledge.”

The proof is definitely in the pudding:

Finally, here’s a YouTube video explaining the concept:

Sum of Three Cubes

I now have a new example of an existence proof to show my students.

Last year, mathematicians Andrew Booker and Andrew Sutherland found solutions to the following two equations: x^3 + y^3 + z^3 = 33 and x^3 + y^3 + z^3 = 42. The first was found by Booker alone; the latter was found by the collaboration of both mathematicians. These deceptively simple-looking equations were cracked with a lot of math and a lot of computational firepower. The solutions:

(8,866,128,975,287,528)³ + (–8,778,405,442,862,239)³ + (–2,736,111,468,807,040)³ = 33

$latex (–80,538,738,812,075,974)3 + 80,435,758,145,817,5153 + 12,602,123,297,335,6313 = 42$

At the time of this writing, that settles the existence of solutions of x^3 + y^3 + z^3 = n for all positive integers n less than 100. For now, the smallest value of n for which the existence of a solution is not known is n = 114.

For further reference, including links to the original articles by Booker and then Booker and Sutherland, please see:

Differentiation and Integration

As I tell my calculus students, differentiation is a science. There are rules to follow, but if you follow them carefully, you can compute the derivative of anything. This leads to one of my favorite classroom activities. However, integration is as much art as science; for example, see my series on different techniques for computing

\displaystyle \int_0^{2\pi} \frac{dx}{\cos^2 x + 2 a \sin x \cos x + (a^2 + b^2) \sin^2 x}

The contrast between differentiation and integration was more vividly illustrated in a recent xkcd webcomic:

Source: https://xkcd.com/2117/