Discriminating determinant

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

What is a determinant?

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.

Determinants are calling me...

Good Story about a class, and my continuing fascination with determinants.

I go to class today, and it is Algebra 2, which I am never really sure how to approach. We are doing a chapter on lines. Now I had all this students last year for Algebra I and it is an honors course, so they are all pretty strong. I come in to class and put the following question on the board:

Determine if the points (-2, 1) (0, 3) and (7, 12) are collinear

And do you know what all the students asked (Go ahead test yourself, see if you can put yourselves in the mind of an 8th grader)

I hope you guessed "What is collinear?" Cause that would be right.

Now we are at the tony private school, so the smartboard is still on from the last class. I login in and go to wolfram.mathworld.com and look up collinear. I let the students read the first couple lines, and now they are off and running. Get lots of good answers/approaches. Some students find the equation of line using two points and check the third point in the equation to see if it works. Some calculate the slope two different ways and see if they are equal. Some calculate the equation two different ways and see if they are equal. It is all very good.

While the students are working I can help but see the following information just a little bit lower down on the mathworld entry for collinear (and this is not a direct quote)

You can check if three points are collinear if with determinants. For points (x_1, y_1), (x_2, y_2), and (x_3, y_3) create the determinant.

|  x_1  y_1  1 |
|  x_2  y_2  1 |
|  x_3  y_3  1 |

if the determinant equals zero then the three points are collinear.

I had totally forgotten about this, even though I had written a post about determinants two months ago.

Now I am standing in front of my class while they work the collinearity problem wondering, should I go for it. Should I show them determinants right now? Should I use this moment to introduce this strange monster of mathematics? It then occurs to me that we can use this to find equations of lines as well, and that in the next chapter I am going to introduce Cramer's Rule. Cramer's Rule is nothing but determinants, and so I go for it. The kids really liked it, and it is a most amazing thing that the series of calculations does this.

The way to find the equation of the line is to take two points and create the determinant:

|  x    y    1 |
|  x_1  y_1  1 |=0
|  x_2  y_2  1 |

this works because you are looking for all the points that are collinear with the other two.

Another way to look at it is the fact that the determinant

|  x_1  y_1  1 |
|  x_2  y_2  1 |*.5
|  x_3  y_3  1 |

Gives the area of a triangle with those coordinates therefore the three points are collinear if and only if the three points define a triangle with zero area.

So the class went well, and they really had an appreciation for determinants. They liked the way this got them pretty close to standard form. They may even remember it. I showed them two by two determinants as well, we will see if they recognize it when it comes up in about two weeks.

So, after another foray with determinants, I left pondering. I wonder "What is a determinant?" What is it's core? It does so many things: linearity, area, invertibility, etc. I just don't know.