Matt-a-matical Thinking

Spirolaterals II

noreply@blogger.com (Matt) — Mon, 06 Sep 2010 14:11:00 PDT

Here I have finally figured out how to post a Excel file on my blog. I know, I am not as quick with the technology as I thought I was. When you open it on sheet one, you can change the turn, numerator, and denominator to see different patterns. There is a large image of the graph on another tab of the workbook.

Enjoy.

Spirolateral.xls

Review of Chocolate Key Cryptography

noreply@blogger.com (Matt) — Sun, 05 Sep 2010 18:46:00 PDT

So this is an idea that has been bopping about in my mind for a while, and while last year was a crazy hectic year for so many reasons, I have worked to make this year less crazy. I want to do some reviews of articles that I read in Mathematics Teacher from NCTM. I could write letters to the editor I suppose, but the feedback loop on those is too short for this web 2.0 iphone instant message world we live in.

So I give you my thoughts on Chocolate Key Crytography by Dale J. Bachman, Ezra A. Brown and Anderson H. Norton in the September 2010 issue Page 100.

The title is intriguing especially in Mathematics Teacher because it promises something that I know very little about called the Diffie-Hellman Key Exchange, so initially I am very interested. Of course it comes with the requisite Mathematics Teacher cute graphics, but at least none of the painful pictures of students with faux engagement painted on their faces.

The article is broken down into several sections. The first covers what cryptography is, including a helpful breaking down of the word into cryptos and graphos for those of you new to the English language. Followed by an explanation that the internet is important and that concepts about cryptography can be taught to anyone, even high school students.

They then go on to describe a fundamental problem of key creation in cryptography. Essentially, you and the person that you want to communicate with have to pick a number together with out meeting. How can you do that? The authors have created a metaphor for this problem that uses M&M's and at that point one of their colleagues probably told them about how math teachers and Mathematics Teacher love M&M's, hence the article.

The article goes on from there to discuss some of the more interesting mathematics of the problem that includes group theory. The multiplicative group of integers . There is also a lot of discussion of how this is not hard and anyone can do it.

Why should we teach it? You may ask, and if you didn't you should ask yourself why you didn't. Well this shows that math has an application in the real world. Finally an answer to the internal question of when will ever use this. Of course, this doesn't answer when will ever use this. Because your students are not really "using" this. Someone else used to create the internet, and as cool as that is, it is something that is done, finished, kaput.

Overall I give it 3 out of 5. It is a nice application of math, but the authors don't give enough explanation or scaffolding to help a teacher present the ideas in the article to their classes. Their tell us that anyone can understand it, but their description is not detailed enough to help, and then they scare away a significant portion of the high school teachers of the world with the discussion of group theory. They also don't do a good job helping teachers to fit this into the curriculum. How could this connect with other topics and questions that high school students have to understand? Still I like cryptography, and I would like to see more computer science and discrete math covered in schools.

Drama and Teaching

noreply@blogger.com (Matt) — Mon, 30 Aug 2010 19:57:00 PDT

Great writing inspires. I have been reading over the last several months a small book that I have found very inspiring. Everytime I pick it up I get new ideas, and I challenge my assumptions about teaching. The most surprising thing about it is the book is not about math, not about teaching, not about students. It is about drama. It is called The Three Uses of the Knife by David Mamet. It speaks to the nature and use of drama.

I know that there is a general sense among many teachers that class does not need to be a show everyday, and clearly that attitude taken to extremes creates teachers that are more like Michael Scott of "The Office" than Dan Meyer.

But Mamet forcefully explains that humans have a dramatic urge. People want to see themselves as part of a giant play with themselves as the hero. As teachers we have a responsibility to engage students, and to ignore this paradigm seems short-sighted at best.

Mamet is very insistent that there are good forms of drama and bad forms of drama. He uses school, politics, the evening news as examples of bad drama carried out every day.

In the next few posts I will reflect on certain sections on Mamet's work and describe what I think it means for teaching. We will start the section he calls The Perfect Game.

Imagine the perfect game. Is it your favorite team thumping their opponent? Of course not, it is your team starting out well, looking dominant and then all of a sudden things fall apart. They change many things and nothing seems to work. Then all of a sudden when things look darkest, a player redeems himself and for some previous screw-up in the game, and scores the go-ahead score, but then the referee calls it back, and again the players must find a way to take the lead, and they do, but the other responds to retake the lead, only to have a more miraculous play occur for your team, and so on and so on.

This is the prototypical three act structure. Humans look for this structure in their lives all the time, and if you watch sports, or follow politics you will see that these activities are recast by commentators in this framework all the time. Mamet would tell us that this recasting is for our pleasure.

Mamet describes this as "Yes, No, But Wait...", and it repeats again and again.

You can imagine this in a classroom, students come in an see a problem something familiar that they know and can deal with. They deal with it easily and look to you for their deserved praise. Then you give them another problem similar, but with some unexpected twist. They work forward trying many things. Many of those ideas fail, and the way seems lost. You don't help them (much, if any) then at some moment through an idea spread through the class based solely off of students previously failed idea (or through some cryptic magic incantation you said under your breath) and the students are off and they solve the problem, or they don't stymied by some other detail they have left out, or you have given them a new problem with a new twist. In this way, we model "Yes, No, But Wait...". And basically we should believe that this is what students want, crave, desire. They do not desire the answer, the algorithm, the process. They desire to be heroes.

Part of what Mamet talks about is our never ending need to dramatize our lives:

For we rationalize, objectify, and personalize the process of the game exactly as we do that of a play or drama. For, finally, it is a drama, with meaning for our lives. Why else would we watch it?

That idea about rationalizing, objectifying, and personalizing is so important. Students need to do that to really connect with the content that we are looking for them to master. If they don't do it this more complex and deeper level, then they will never remember it.

Spirolateral Break

noreply@blogger.com (Matt) — Sat, 16 Jan 2010 21:08:00 PST

Spirolaterals are an idea that I don't remember where I read about them, but the idea is relatively simple. You take the the decimal expansion of some fraction and using the digits create segments of those lengths then rotate some fixed number of degrees. I have an Excel spreadsheet that creates these pictures. If I figure out a way to post the file I will. Let me know if you know of a way to do that.

Here is 8/147 with a 90 degree turn at each step.

Math Elective

noreply@blogger.com (Matt) — Fri, 08 Jan 2010 19:59:00 PST

I have been reading a number of the posts on David Bresoud's blog at maa.org. They are slowly bringing me to the conclusion that my profession is in danger. More and more I see math education mirroring the american automotive industry. We are certain that there will always be a demand for what we provide. We are certain that people will desire a version of that product that is nearly identical to version we produced 20 years ago. This may not be true, and we have gone a long way to "reform", but there is still a long way to go. Bresoud articles seem to point out consistently our ridiculous adoration of calculus as the bridge between high school and college mathematics. He points out also how this leads to all kinds of systems and policies that lead people to take math they don't want and they don't need. The math they do want and do need is often under-supported, under-credited, and under-appreciated.

Those are my interpretations of his articles. You may have your own. One of his more recent posts, here, talks about the indicators of success in college. And of course points out what many know, the SAT and ACT are not particularly good predictors of college success. Which is what I understood their importance to be in the first place.

So what if SAT and ACT went away?

At first, I was excited by the prospect. So many pieces of ridiculous mathematics could be jettisoned from the curriculum. We would have freedom to create programs that make sense for today. It would be easier to rest control of the curriculum from calculus and refocus on statistics and discrete math. More and more students are going in biological sciences (need statistics) and fewer and fewer are becoming engineers (need calculus).

Then the dark side of it hit me. If math teachers could say you need this for SAT, then what is the likelihood we could keep our requirement status. Would you really need four years. How many students and parents would like their child to not HAVE to take math. Based on how many people regularly tell me that they hate math or were never any good at it, A LOT.

And what if happened quickly. Where would we be?

This is why I feel we have to teach math as if it were an elective. Every class. Every class needs to have a clear purpose. Students should not leave high school without knowing how to use excel, because if you are going to do real math in the real world you are going to use excel at some point. So screw the calculator and get the computer.

How much data is being produced today? Way more than can be analyzed currently, but they are looking for people to do it. Why has AP Statistics grown and grown. When will it plateau?

I guess I have a lot of questions, but I don't know how to rattle the colleagues I see around me to the coming danger.

Visualizing Numbers Real and Imaginary

noreply@blogger.com (Matt) — Fri, 24 Jul 2009 12:28:00 PDT

In the video below I try to give some of the "patter" I use when I introduce the idea of "i" for the first time. I think that there is a great benefit in doing it this way because it helps strengthen their understanding of what real numbers do as well. It also emphasizes that numbers often get paired with operations. I actually could make that clearer. Anyway, feel free to steal this introduction to use in your own classes.

Visualizing Complex Numbers

noreply@blogger.com (Matt) — Thu, 23 Jul 2009 21:16:00 PDT

In my continued quest to spread an understanding of complex numbers I put together this little dance sequence with my Summer School Algebra 2 students. I used the function f(x)=(x-1)^2+1 determine the dance sequence. This function has two complex roots (1+i) and (1-i). I had students stand at 1+i, i, -1+i, 1, -1, 1-i, -i, and -1-i, then we went through the three steps of the function. These were "minus 1", "squared", and "plus 1". The most important visual here is to get a sense of what squaring does to the complex plane. This includes some expansion of the numbers with distance greater than 1 and a wrapping of plane on top of itself. Watch the video and let me know what you think?

How is a function like a recipe?

noreply@blogger.com (Matt) — Tue, 21 Jul 2009 07:57:00 PDT

I was talking about function notation with my students, and trying hard to differentiate between f, f(x), and f(x)=x+3. The metaphor that I tried was recipe. Does anyone else have a good metaphor that helps to distinguish these concepts?

Sneetches = Inverse Functions

noreply@blogger.com (Matt) — Tue, 21 Jul 2009 07:54:00 PDT

Teaching summer school today. I had a student not understand what f(f^(-1)(x))=x was trying to say. I was searching for a process that would help her understand doing and undoing. And then it hit me. Sylvester McMonkey McBean.

Conic Movie

noreply@blogger.com (Matt) — Thu, 11 Jun 2009 19:38:00 PDT

I have been reading the book Conics by Kevin Kendig. This movie was made with Grapher on my Mac, and displays the viewpoint of taking conics and projecting them down on the sphere.

Make Complex Numbers Real

noreply@blogger.com (Matt) — Wed, 18 Jun 2008 19:21:00 PDT

This post is continuation of this rant.

We do such a bad job with complex numbers that students often assume that the word Imaginary in front of Number is an adjective that i is not really a number. It is some sort of imaginary thing.

One important reason that students find this easy to believe is the fact that it is not possible to order the complex numbers. Ordering is a basic and fundamental concept for numbers. Ordering is first thing that students are to learn about numbers. We stress the idea of ordering of the numbers, we use the ordering of numbers to help explain addition and subtraction. So if we can neither tell if i is neither bigger or smaller than 0, then it might not really exist.

So makes imaginary and complex numbers real. What makes them real is that can represent things that happen in the real world. Real numbers can represent length and direction (direction in one dimension, left or right, up or down). Complex numbers can represent length and direction in two dimensions. So 3+4i can represent 3 steps to the right and 4 steps up. You can represent movement in two dimensions with complex numbers. But that is not different than vectors. What makes complex numbers so different is the existence of a coherent multiplication rule, one that even allows a coherent definition of division.

So it is multiplication that makes complex numbers so special. What does multiplication in complex numbers do. It does two things, it is capable of doing what multiplication of real numbers does, which is dilation. Multiplication by a positive real number performs a dilation on the points of the complex plane. Multiplication by a negative real number performs a dilation and a 180° rotation. In particular, multiplication by -1 is just a 180° rotation. Now how about multiplication by i. If you take any complex number and multiply it by i the result is a 90° rotation about 0. And from all that we know about i, this makes tremendous sense. We know that i is the square root of negative one, so that multiplying by i does half of what multiplying by -1 does. We know that i^4=1 so that multiplying by i four times does nothing to a number, just as rotating by 90° four times returns things to where they started. It turns out then that multiplying by complex numbers models rotation! This is what is important. Turns are everywhere. Everywhere in life there are rotations of all sorts and these numbers allow you to do computations with coordinates that affect the desired rotations. Want to turn 90°, multiply by i. Want to turn 45°, multiply by sqrt(i). This is why polar form for a complex number exists, and it is the importance of DeMoivre's Theorem.

As a final note tonight. Let's bring it back to algebra. Let's say we have to solve x^6=1. Then we are looking for 6 solutions. Well then these are complex numbers that multiplication is the same as rotating 0°, 60°, 120°, 180°, 240°, and 300°. In other words 1, 1/2 +sqrt(3)/2 i, -1/2 +sqrt(3)/2 i, -1, -1/2 -sqrt(3)/2 i, 1/2 -sqrt(3)/2 i. But what is better is seeing it on the complex plane.

That is right the solutions to this equation are vertices of regular polygon, and more this generalizes. So that the regular polygons can be associated to the polynomials x^n-1.

Why do we make it so complex???

noreply@blogger.com (Matt) — Tue, 17 Jun 2008 21:32:00 PDT

Are you a math teacher? If you are then you probably have built up a tolerance to the funny looks; a tolerance to the subtle comments that are meant to infer that you are VERY different from everyone else. You must have this tolerance or how else could you continue in your profession. I understand. I have it too.

But here is the thing. It dulls us to the messages around us. Something is wrong in in the state of math education. Most of us know this, though some still fight it. Most of us know that it is the charge of math teachers to make it better. We must raise an entire generation of children that meet math teachers and say, "Yeah, math was always too easy for me so I decided to do something difficult like teach grammar instead." That is our charge, and to make it happen we need to assiduously evaluate everything that we do. We must find the mistakes and fix them. I don't know if what I am about to propose a solution to is the most pressing problem, but it definitely is one. We may be able to learn something from the problem, and I hope something from my proposed solution. Here goes...

Complex Numbers

Let's take what a child is supposed to know about complex numbers after Algebra 2. Algebra 2 is or was a terminating course in high school math. So this may be all that any one every learns about these crazy things. Here are the objectives from the Algebra 2 book I am teaching out of during summer school:

Simplify radicals containing negative radicands
multiply pure imaginary numbers
solve quadratic equations that have pure imaginary solutions
add, subtract, and multiply complex numbers
simplify rational expressions containing complex numbers in the denominator

Wow. How incomplete a person would feel if they had not accomplished these feats of learning.

This is really the first time that students have been exposed to these kinds of numbers. Some have heard about them and wondered what they meant. And what do get to find out about them. That the purpose of complex numbers is to be added, multiplied, subtracted and simplified. Imagine that you were trying to sell and innumerate person on the integers.

Stone Age Math Teacher: Hey, have you heard about this great new number -1?

Stone Age English Major: Cool what can you do with it?

Stone Age Math Teacher: You can add, subtract, multiply and divide with it. Pretty cool huh? And you can solve any subtraction problem if you allow a whole class of new numbers called the integers!

Stone Age English Major: Wow. Major. Let's go invent beer, so I can invent poetry.

A purpose for complex numbers must be the ground work to any introduction to these numbers. Many of you may feel that these numbers have no purpose. Or that maybe the purpose is to allow us to say that yes, every quadratic does have two solutions (if you count multiplicities). But no, the purpose is real. It is part of our everyday existence. Just as the reason that 1, 2, 3 came about was to help sheep herders keep track of the flock. We are talking real.

What is it. Yawn.... I will write that tomorrow.

Cat's (Synthesized) Meow!

noreply@blogger.com (Matt) — Sun, 15 Jun 2008 20:40:00 PDT

Our latest post on determinants made the carnival of mathematics at catsynth.com. Which reminds me of one of the things that makes me happy in the world... Cats falling compilation videos on youtube. One of my AP Calc BC kids showed me this one. Those kids are so smart.

I really love the music. I always wondered what the purpose "Man in motion" was.

Discriminating determinant

noreply@blogger.com (Matt) — Fri, 13 Jun 2008 21:20:00 PDT

So far we have found that we can use the determinant to find area and volume, and that they can also help us reform the concept of proportion. Determinants can provide a way to calculate this quantity in some cases where other methods could be much more difficult. We have seen that the determinant is a generalization of proportion. What more?

One of the most important properties of determinants is that if two rows (or columns) are the same then the determinant is zero. This can be easily seen from the area interpretation of the determinant, because if two columns are the same then the area defined is degenerate for that dimension. The box has been flattened.

This is very similar to another way in which we discriminate things in math. In algebra we often see the quantity (x-a). This comes up in factoring or even in creating polynomials that have specific quantities, such as Lagrange Interpolation.

Using determinants to discriminate

Just like the expression (x-a) has the ability to test if the value of the variable x is equal to a, Discriminant will allow us to test if many variables have desired quantities simultaneously.

Example: Finding the equation of a line

In this example uses the multi-linearity of the determinant to find the equation of a line through two points. For example let’s try to find an equation of a line through (3, 5) and (-2, 3). To do this with determinants we set up the following 3x3 determinant equation.

Clearly, the points (3, 5) and (-2, 3) satisfy this equation, so that solution set does go through those two points. Is it a line? Well from any method that you choose to evaluate the determinant you can see that the coefficient of x and y will be numbers. Therefore it is a line.

Why are the in the third column? Well we need to make sure that we have a square matrix to take the determinant of, and we need to make sure that the three rows have the same number. So that we can get the determinant to evaluate to zero when we insert the points we know.

Example: Equations for other curves

One problem that we often ask students in Algebra 2 is to find the equation through three points. Let’s do one now, for instance find an equation through the points (3, 5), (-2, 3), and (6, 9). Then we set up the following determinant equation.

The numbers in the first column are the squares of the numbers in the second column. In fact, we can find an equation for a circle through those three points by the following equation.

Conclusion

Determinants may have more use and meaning that we have given them credit for in the standard high school curriculum. Can we bring them to the students in a meaningful way? Can we show them their usefulness and their beauty? I encourage you to explore this with students, friends

Schlope and PMAN

noreply@blogger.com (Matt) — Thu, 29 May 2008 21:38:00 PDT

Mathematics is the art of giving the same name to different things. - Henri Poincare

So in this case it is not the quite the same name, but I have sort of "invented" some terminology. The idea of "schlope". Schlope is quite similar to slope. The slope of a line tells you how much to go up for every 1 that you move to the right. Schlope tells you what to multiply by for every one step to the right. Schlope is very important for exponential functions. Generally, in exponential situations we talk about the "rate of growth". If the rate of growth is 8% then the function that models that growth has "1.08 to the t" in it. That 1.08 is what I call "schlope". So, schlope is the number that you actually use in the formula.

In fact, you can create parallels for all your favorite formulas and constructs from linear functions. This parallel construction highlights another construct of mine that I like to call the hierarchy. The hierarchy is simply the recognition that the fundamental ways of putting numbers together come in an order. At the bottom level we have Addition and Subtraction. In the middle we have Multiplication and Division, and at the top powers, roots, and logarithms. Another teacher at my school refers to this as "PMAN" for powers, multiply, add, nothing. It is very helpful to think about PMAN when doing calculations with exponents and logs. If you are taking powers of powers, "P", then you "M"ultiply the powers. If you are "M"ultiplying powers with the same base then your "A"dd the exponents. If you are "A"dding different powers of the same base together then you do "N"othing.

To move from Linear functions to Exponential functions you simply must move up in the hierarchy. Moving from repeatedly adding a number to the height every time to multiplying that height by some fixed number. The formulas can be written quite similarly:

Highlights Highlight

noreply@blogger.com (Matt) — Wed, 28 May 2008 22:27:00 PDT

My wife and I wrote an article in Highlights magazine about factorial. You probably know the magazine from the doctors office. You can read the article here Big numbers at a banana-split bar but you miss all the pictures. Guess you will have to find it at the dentists office.

Are quadratics really that important

noreply@blogger.com (Matt) — Wed, 28 May 2008 07:33:00 PDT

If I was a student who had just finished a course in Algebra I in most schools across America I think that I would think that the following were the most important things that we had done. I would think this because we had spent so much of our efforts on it.

linear equations
distributive law
quadratic equations
factoring

The problem with this list isn't so much what is on it, it is what isn't on it. It seems to me that we simply must include exponential functions and therefore also logarithms. I guess my question is, "Are quadratics really that important?"

The only way to add these topics in is to take other topics away. So what can go? Here is a partial list of topics that might be cut or reduced in the standard curriculum.

completing the square
conics
rational root theorem
long division of polynomials

Exponentials seem like such a natural topic, and one that could be made to support and build on linear equations so beautifully. I will give more of this idea when I write later about "schlope".

Determinants as proportions

noreply@blogger.com (Matt) — Sun, 25 May 2008 20:21:00 PDT

This is a continuation of a discussion of determinants started here. And there is more here.

Proportions

So if determinants are important how important are they and when can they be introduced reasonably. One place that they could possibly come up is in reference to proportions. As a reminder, a proportion is equation where two ratios are equal. One method to solve equations such as this is the Means-Extremes Property. This is more commonly known as “Cross-Multiplying”. I think I speak for many teachers that cross-multiplying is a “bane of existence”. Cross-multiplying is a rule that is often overused. It seems to quickly rise to the top of all students list of favorite methods so that whenever in doubt about how to proceed in a problem with fractions teachers often here the idea put forth that the correct method might include cross-multiplying. This is probably because few students really understand what this method does or why it works

Connections between determinants and proportions

So what is the connection between determinants and cross multiplying. Well
it can be seen from a variety of ways. First is in the formulas themselves. A
proportion has the form:

after cross-multiplying we know that

this last equation can be rewritten in terms of determinants as

In a proportion we are given that two fractions are the same. Each of those fractions can be thought of as vectors. Similar to the definition of slope as a fraction or as a vector. With this definition of the fraction we see that the two fractions will be equivalent if their vectors point in the same direction. If they point in the same direction then the area given by the determinant will be zero.

Advantages of the Determinant Formulation

The advantage of this determinant method to solving proportions is that we eliminate the fractions from the problem. Cross-multiplying would only exist in determinants, where the rightly do play a role. Students would be less likely to misapply the idea of cross-multiplication to every situation with fractions. Determinants would be introduced earlier and their presentation of area would be well supported. Students would also have to have a clearer understanding of slope as a primary way of looking at fractions.

What is a determinant?

noreply@blogger.com (Matt) — Wed, 14 May 2008 20:01:00 PDT

Determinants pros/cons
Determinants, for a teacher if I may borrow a British phrase, they are a bit of a sticky wicket. For my teaching, determinants have the qualities that I find makes a topic uninviting for students:

➢ The rules for manually calculating determinants seem arbitrary, and are nearly impossible to motivate.
➢ It is not clear what the determinant means. By which I mean it is not clear what “natural” meaning might be given to a the value of a determinant, in much the same way that slope can be imbued with the natural meaning of “rate of change”.

But determinants are also tempting:

➢ Cramer’s rule presents a powerful way to solve systems of equations
➢ Matrices are a vast generalization of number systems that have far reaching applications. Can we use determinants as a way to motivate the study of these objects?

When teaching we need motivations
The first question I ask about a topic, is what might lead someone to want to know this? Why have “slope” or a “determinant”. For motivation, slope benefits from the universality of change. A recurrent theme in mathematics education from algebra to calculus is the wide range of applications of the slope concept to measure change.
Each child must see the motivation in mathematics
History of math as a motivation
What can we use to motivate determinants? Often when I find myself struggling to motivate something I will study the historical motivations of a topic. For instance, the rules of probability are often well motivated by the story of the way Fermat and Pascal were lead to discuss a game that motivated many questions about probability.
The determinant’s history may not be as storied. It begins to late in the history of mathematics to have a wonderful story attached. Professional mathematicians, not interested lay practitioners brought it forth.
Connections/Generalizations of previous curriculum
To be a topic worthy of our student’s interest it must be made to represent something meaningful in their lives, in their experience, or in the larger life of the mind. Can we at least show that a curious soul would want to see what this is all about? We can only do this if the determinant can tell us something that our students find real. Rules that utilize determinants in part, such as Cramer’s rule, are inadequate for this job because the determinant is only part of the equation. Why not make the whole thing one giant new equation? We need to start with unadorned determinants to begin with.

In the rest of this article I will try to make the leap. I will try to motivate determinants and demystify them, and I will indicate some more uses for them. The more we understand about the determinants the more our students will understand them and find even more remarkable ways to use them.

Volume and Area
Volume and Area do provide a very concrete motivation. It is clear to a student that measuring the size of objects is a fundamental question of existence. There are two formulas for area that I would like to focus on. These are the standard the equations for the area of a parallelogram and the area of a triangle.

The inadequacies of some formulas for some area

These formulas are absolutely correct and deeply flawed. The major drawback of these formulas is that they rely on a length that is not truly “part” of the shape. The height measurement would in reality be a difficult measurement to determine. How is it possible to determine that we are really are measuring an altitude?

Determinants provide another method to find area. In this version we must assume that the vertices have been given coordinates. Then it is possible to find vectors that represent the sides. These vectors not only encapsulate the length of the side but also the direction. It is therefore possible to find the area of a parallelogram or triangle from these measurements.

For instance, take the parallelogram shown below.

In this case we can compute the vector from A to B is <> and from A to D is <>. Using these vectors we can compute the determinant and from that the area.

Because we are interested in computing with vectors we can in general imagine that one of the corners of the parallelogram is at the origin. This gives us a figure like the one below.

We can then compute the area of the parallelogram by finding the area of the rectangle and then subtracting the area of the green trapezoids and orange triangles.

This shows that the area of the parallelogram can be computed from the vectors that define the parallelogram. Because triangles are half the size of a parallelogram we can compute the area of triangles similarly.

Where and are the vectors that define two adjacent sides of our shapes.

This analysis can be extended to parallelepiped in three dimensions.

Next, we look at applying the determinant to problems involving proportions.

A tease

noreply@blogger.com (Matt) — Mon, 18 Feb 2008 19:15:00 PST

I keep forgetting about all the things I want to write about. So I will make a public list and hopefully these posts will come in quick succession.

Tchebyshev Polynomials (Thanks to Al Cuoco again)
Combinatorial Game Theory textbook
Blogging as an assignment
The genius of Euler

Feel free to write in the comments that one of these is more interesting than the next.

More Determinants

noreply@blogger.com (Matt) — Mon, 18 Feb 2008 19:12:00 PST

I was at the Joint Mathematics Meeting in January, and their I met an author that I really enjoy. His name is Al Cuoco. He does a lot with mathematics education and his book on Mathematical Connections is a must for any high school math teacher who wants to give their students a hint of the beauty of mathematics beyond high school math. I highly recommend it.

Since he was there, and he was speaking a little bit about linear algebra, I decided to ask him the question that has been bothering me for at least a year. What is the the determinant? Remember from my previous posts that it is not that I don't know how to calculate a determinant, or when to use a determinant. What I don't know is how to explain it to students. How to give it meaning. Al's opinion appears to be that it is most clearly connected to volume. To help prove his point he sent me a graphic which previous appeared on the cover of a math journal. The graphic does a pretty good job of explaining how determinants can be used to find the coordinates in a different coordinate system. It basically comes down to projections.

Pretty cool. I think that I will try to tie together the different determinant thoughts that I have had over the year in one post. I will hopefully be able to add some of the things that I have learned in Al's papers.

Calculator programming

noreply@blogger.com (Matt) — Thu, 14 Feb 2008 20:46:00 PST

While covering the quadratic formula with my 8th grade algebra students, I decided to show them how to program their calculator to do the quadratic formula. Students had a surprisingly strong response to it. They loved it.

Certainly they love the power of having the quadratic formula in their calculator. And their may be less chance that they will memorize the quadratic formula because they don't have to do it as often as they would have had to if I hadn't given them the formula. But it was a great way to discuss the ideas of input and output. I drew a function machine on the board, and we discussed how other machines can be seen as functions. So this connects this fundamental mathematical concept of function for them to their lives, and it allows them a way to construct a function of their own. We did a problem of finding how long it takes for an object to fall a certain distance. Students really started to get it.

In fact, after we programmed that we wrote a program to find the vertex, both the x and y coordinates. This was great for them because it emphasized that this process of writing a computer (calculator) program is really whatever you want. Got a process that you are doing over and over then make a program. I also indicated that these two separate programs might be merged together to create a bigger more complex program that could do both things. Other teachers told me later that the students were still talking about it several periods later.

On the technology side, it really helps to do this with a smartboard and TI-smartview. I was asked so rarely about what did I type or where that button is on the calculator. It is such a relief to have these tools.

Bear hunting Algebra

noreply@blogger.com (Matt) — Sun, 10 Feb 2008 20:50:00 PST

I am finding myself more of a shameless shill for mathematics in my 8th grade Algebra class. Want me to jump around like an idiot? I will do it. Want me to make a snowball and bring it in to the classroom? Good as done. So you can see I am interested doing anything that will meet my students where they are...

One more crazy thing I have been doing is trying to connect the kids books I read to my math class. And I think I found out one that actually worked a little bit. It is Going on a Bear Hunt...

You may the know story, you may have been to camp, you may have never heard of it, which may mean that you have never had a child, been to any place where teenagers are asked to supervise young children. If that is you click here to get a sense of what it means to go on a bear hunt.

I use this to remind/reinforce the idea about doing and undoing to solve an algebraic equation. So with the equation:

3x+1=10

I say we are going on bear hunt we are going catch a big one we are not scared. Oh no a times 3!. Can't go over it can't go under it... Oh no a plus 1, etc. And then I said a 10! AAAAAAHHHHHH! Back through the 1 (minus 1), back through the 3 (divide by three). These are the steps (minus 1 and divide by 3) that you need to solve the problem.

Now as many of surely know, much of this is not "necessary", but my students need the background of reading through the problem the first time to understand the order of operations. In fact, many of my students still don't understand that the 3x cannot be undone by subtracting 3.

I hope that this helps them see not only what to do, but why they are doing it.

At the very least I am exposing them to classic literature, well literature, well words.

Carnival of Education

noreply@blogger.com (Matt) — Tue, 27 Nov 2007 22:00:00 PST

As we approach the end of the year will we be inundated with end of the year awards, and of course not so recently we had the Nobel Prizes. Named after Alfred Nobel the inventor of TNT (Boom!). Well here at the 147th Carnival of Education we are going to have our own prizes. The Noble Prizes, these prizes will celebrate that noble avocation to which we have all taken the time to consider: Education. Education is defined by Miriam-Webster to be:

... 2 a: to develop mentally, morally, or aesthetically especially by instruction b: to provide with information : inform ...

So to all of you that have made Education your passion even for only this week, Thank You, here is a prize.

The first Noble Prize goes to one of my favorite teachers growing up, Brinda Price. She was my ninth grade English teacher. As I remember her she was about 4 feet 10 inches, and she was tough as nails. You had better get your Romeo and Juliet packet in on time or else she wasn't going to take it. On the other hand, she loved us. She loved us with a mother's love. She wanted us to grow, and live, and enjoy life. She had a passion for her subject that was only exceeded by her passion for her students. She is one of my heroes, in the sense that I want to make my students feel the way I did in her class. I want my students to see me as a force of nature that cares about them and about math and mostly about their education. I desperately, forcefully, passionately want them to develop mentally, morally, or aesthetically especially by my instruction.

So the first Noble Prize goes to Brinda Price of Columbus Alternative High School circa 1984. Thank you, and I am sure that the lack of prize money will not surprise you given your choice of profession.

The Noble Prizes are given to recognize the sacrifice that we all make to our charges, you are all working to make the future better, to make each child's lives better.

The Noble Prizes for Valiant Effort go to Henry Cate for Public schools - a Gordian Knot or a Sisyphean activity?, Mike Cruz for Losing Difficult Students - Blessing or Loss?, and Mrs. Bluebird for Playing Principal.

The Noble Prize for Growing and Becoming More Zen so that Your Students Can Have a Good Day goes to Siobhan Curious at small tasks.

The Noble Prize for Improving the Profession goes to Bill Ferriter for The PLC Mandate. . ..

The Noble Prize for Literature goes to Rebecca Wallace-Segall for Schools: Celebrate Teen Writers and Lessons from the Newest Generation of Writers (& Thinkers).

The Noble Prizes for Teaching Resource Coordination go to ms. teacher for Sharing!, Joel for 50 Classroom Management Tips I Have Learned This Month, and Ryan for Bridging the Research-Practice Gap.

The Noble Prize for Opening a Window into a Soul goes to IB a Math Teacher for The Book of Me.

The Noble Prize for Unschooling goes to Laureen for Bucket-Free.

The Noble Prizes for Elementary Education go to What It's Like on the Inside for The Sad State of Elementary Science, and Ryan for How Kindergarten Has Changed.

The Noble Prize for Anti-Telepathy goes to Mr. Pullen for Hey, Kids: Guess What I'm Thinking!.

The Noble Prizes for Public Policy go to Judy Aron for Tax Credits For Homeschoolers - Bad Idea!, EdWonk for EduDecision 2008: Obama's $18 Billion EduFix?, Joanne Jacobs for Defining dangerous down, and Dave Saba for Math: there is no substitute | American Board for Certification of Teacher Excellence.

The Noble Prizes for Math go to Denise for Fraction models, and a card game, Tony Lucchese for A Letter to a Young Mathematician, and Matt (that's me!) for Lessons on Lessons While Cooking Mashed Potatoes.

The Noble Prize for Art goes to Scott Walker for Some sketches during a staff development session.

The Noble Prize for Thoughtful Homeschooling (is there any other kind?) goes to Dana at A workable solution for American education.

The Noble Prize for Film Advertising goes to Matthew K. Tabor for his highlighting of a Screening of 2 Million Minutes.

While the Noble Prizes for Film go to Larry Ferlazzo at Math Movies and More, and Adam for The Academic Schools.

The Noble Prizes for Humor go to mister teacher for Helpful or Harmful, Smellington G. Worthington III for Welcome, and Carol Richtsmeier for Lists, Parents & Paperwork.

The Noble Prize for Zoology goes to Ms. Cornelius for Wanted: One case of mouse-sized Depends Undergarments.

The Noble Prizes for NYC Education go to Norm Scott for UFT Candlelight Vigil Snuffed, Woodlass for Sacrificing the learning years — Why?, and NYC Educator for his accounting of 7.2 million dollars in What A Bargain!.

The Noble Prize for Statistics goes to Edwonkette at Lies, Damned Lies, and NAEP Exemptions.

The Noble Prize for Networking goes to Pat for Networking is Important for All Teachers.

The Noble Prize for Civics goes to Matt Johnston for Not Every Education Problem Begins and Ends at NCLB.

Then Noble Prize for Homework goes to michele lestage for Too Homework Much Help Can Result In Failure.

The Noble Prize for Identifying Hypocrisy goes to Right on the Left Coast for Teachers in My District Say Teachers Don't Care About Students.

The Noble Prize for Foreign Language goes to Maria Fernandez for Free Spanish online lessons on mp3.

The Noble Prize for British Education goes to oldandrew for The Two Discipline Systems.

The 148th Carnival of Education will be at So You Want to Teach. Entries are due at 5pm Central on December 4th.

Reading the submissions was a great honor. I learned a lot, and that is what is all about. Thanks to all our contributors for their thoughtfulness and their giving hearts.

Lessons on Lessons while Cooking Mashed Potatoes

noreply@blogger.com (Matt) — Thu, 22 Nov 2007 21:03:00 PST

I hope you had a wonderful Thanksgiving. We had a wonderful time here. We didn't go anywhere we stayed home, my Mom came, and we cooked here. As often happens, while discussing making mashed potatoes with my wife, I had a little epiphany. Maybe it isn't an earth shattering discovery, but I love a metaphor and I think this is a good one, so bear with me and I think you find a story that all teachers can use with their students.

As I said we made Thanksgiving dinner here this year. My wife picked the recipes, and it just happened that we picked all of our recipes from a cookbook we have from America's Test Kitchen. One of the things that we love about this cookbook is that it tells you their theories about "why" they do things. My wife were talking particularly about the mash potato recipe. I am sure that many of you know this, but it is important to add the butter BEFORE the milk. This has to do with "coating starches, etc., etc.", and my wife noted that it was not something she knew about. I told her that I had shared that same information, about the butter before the milk, with my wife's mother. My mother-in-law seemed to think that this was not interesting news of any sort, but something everyone knew. Of course, her own daughter didn't know it.

What does this have to do with teaching math. We often show students what to do. And often we are surprised by the ways they fail to do what we show them. But in this example my wife watched her mother make mashed potatoes many times, but because she didn't know why the butter went in before the milk she didn't know that there was any importance to the order. Since then, my wife has been mixing mashed potatoes, milk, and butter all at the same time. The Horror.

Well the mashed potatoes were great, and I believe that my classes will be improved because I have been reminded one more time that the model of teaching math where you ask students to just do what you do, and not show why is forever deeply flawed. Why is it flawed? Because it leads to lumpy mashed potatoes.

More secrets of mashed potatoes here.