# Dan Meyer says JUMP and I shout HOW HIGH?

On a recent blog post, Dan Meyer professed his love for me. He did it in his own way, through his sweet dulcet tones, declaring me a reality TV host and a Vegas lounge act [1]. LOVE!

He was lauding a worksheet… well, a single part of a worksheet… I had created. You see, I’m teaching Precalculus for the first time this year, and so I have the pleasure of having these thoughts on a daily basis:

What the heck are we teaching this for? IS THERE A REASON WE HAVE KIDS LEARN [fill in the blank]? WHAT’S THE BIG IDEA UNDERNEATH ALL OF THIS STUFF?

[Btdubs, I love teaching a new class because these are the best questions EVAR to keep me interested and to keep my brain whirring!]

And I went through those questions when teaching trig identities. And so I concluded the idea of identities is that two expressions that look different are truly equal… and they all derive from a simple set of ratios from a triangle in a unit circle. Equivalent expressions. When things are the same, when things are different…

So my thought was to make graphing central to trig identities. For the first couple days, every time kids were asked to show an identity was true, they were asked to first actually graph both sides of the equal sign to show they truly are equivalent. (And half the time, they weren’t!)

[If you want, my .doc for the worksheet above is here... and the next worksheet with problems to work on is here in .doc form too.]

To be honest, I still have some thoughts about trig identities that I need to sort out. I am still not totally satisfied with my “big idea.” I still have the “so what” banging around in my brain when thinking about equivalent expressions. I have come to the conclusion that the notion of “proving trig identities are true” is not really a good way to talk about proof. There’s also the really interesting discussion which I only slightly touched upon in class: “Are $1$ and $\frac{x}{x}$ equivalent expressions?” I have something pulling me in that direction too, saying that must be part of the “big idea” but haven’t quite been able to incorporate.

If I were asked right now,  gun to my head to answer, I think I suppose I’d argue that “big idea” that a teacher can get out of trig identities are teaching trial and error, the development of mathematical intuition (and the articulation of that intuition), and the idea of failure and trying over (productive frustration). Because I think if these trig identities are approached like strange mathematical puzzles, they can teach some very concrete problem solving strategies. (To be clear, I did not approach them like strange mathematical puzzles this year.) Now the question is: how do you design a unit that gets at these mathematical outcomes? And how do you assess if a student has achieved those? (Or is truly being able to verify the identity the fundamental thing we want to assess?) [2]

[1] Except I got my teaching contact for next year, and I’ll be making more than the tops of those professions combined. YEAH TEACHING! #rollinginthedough

[2] Different ideas I remembered from a conversation on Twitter… Teachers have contests where they see how many different ways a student/group/class can verify an identity. And another idea was having students make charts where they have an initial expression, and they draw arrows with all the possible possibilities of where to go next, and so forth, until you have a spider web… What’s nice about that is that even if students don’t get to the answer, they have morphed the original expression into a number of equivalent and weird expressions, and maybe something can be done with that? I also wonder if having kids make their own challenges (for me, for each other) would be fun? Like they come up with a challenge, and I cull the best of the best, and I give that to the kids as a take home thing? Finally, I know someone out there mentioned doing trig identities all geometrically, with the unit circle, triangles, and labeling things… I mean, how elegant is the proof that $\sin^2(\theta)+\cos^2(\theta)=1$? So elegant! So coming up with equivalent expressions using the unit circle would be amazing for me. Anyone out there have this already done?

# Families of Curves

When I put out my call for help with Project Based Learning, I got a wonderful email from @gelada (a.k.a. Edmund Harriss of the blog Maxwell’s Demon) with a few things he’s done in his classes. And he — I am crossing my fingers tight — is going to put those online at some point for everyone. To just give you a taste of how awesome he is, I will just say that he was in NYC a few years ago and agreed to talk to my classes about what it’s like to be a real mathematician (“like, does a mathematician just like sit in a like room all day and like solve problems?”), and have kids think about and build aperiodic tilings of the plane.

Anyway, he sent me something about families of curves, and that got my brain thinking about how I could incorporate this in my precalculus class. Students studied function transformations last year in Algebra II, and we reviewed them and applied them to trig functions. But I kinda want to have kids have some fun and make some mathematical art.

First off, I should say what a family of curves is.

That’s from Wikipedia. A simple family of curves might be $y=kx$ which generates all the lines that go through the origin except for the vertical line.

I made this in Geogebra with one command:

Sequence[k*x, k, -10, 10, 0.5]

This tells geogebra to graph $y=kx$ for all values of $k$ from -10 to 10, increasing each time by 0.5.

Okay, pretty, but not stunning. Let’s mix things up a bit.

Sequence[k*x+k^2, k, -10, 10, 0.1]

Much prettier! And it came about by a simple modification of the geogebra command. Now for lines with a steep slope, they are also shifted upwards by $k^2$. This picture is beautiful, and gives rise to the question: is that whitespace at the bottom a parabola?

Another one?

Sequence[1/k*sin(k*x),k,-10,10,0.2]

And finally, just one more…

Sequence[1/k*tan(x)+k,k,-10,10,0.1]

Just kidding! I can’t stop! One more!

Sequence[k sec(x)+(1/k)*x,k,-10,10,0.25]

What I like about these pictures is…

#### THEY MAKE ME WANT TO MAKE MOAR AND MOAR AND MOAR

And then, if you’re me, they raise some questions… Why do they look like they do? What is common to all the curves (if anything)? Does something special happen when $k$ switches from negative to positive? What if I expanded the range of $k$ values? What if I plotted the family of curves but with an infinite number of $k$ values? Do the edges form a curve I can find? Can I make a prettier one? Can I change the coloring so that I have more than one color? What would happen if I added a second parameter into the mix? What if I didn’t vary $k$ by a fixed amount, but I created a sequence of values for $k$ instead? Why do some of them look three-dimensional? On a scale of 1 to awesomesauce, how amazingly fun is this?

You know what else is cool? You can just plot individual curves instead of the family of curves, and vary the parameter using a slider. Geogebra is awesome. Look at this .gif I created which shows the curves for the graph of the tangent function above… It really makes plain what’s going on… (click the image to see the .gif animate!)

Okay, so I’m not exactly sure what I’m going to do with this… but here’s what I’ve been mulling over. My kids know how to use geogebra. They are fairly independent. And I don’t want to “ruin” this by putting too much structure on it. So here’s where I’m at.

We’re going to make a mathematical art gallery involving families of curves.

1. Each student submits three pieces to the gallery.

2. Each piece must be a family of curves with a parameter being varied — but causing at least two transformations (so $y=kx^2$ won’t count because it just involves a vertical stretch, but $y=k(x-k)^2$ would be allowed because there is a vertical stretch and horizontal shift).

3. At least one of the three pieces must involve the trig function(s) we’ve learned this year.

4. The art pieces must be beautiful… colors, number of curves in the family of curves, range for the parameter, etc., must be carefully chosen.

Additionally, accompanying each piece must be a little artists statement, which:

0. Has the title of the piece

1. States what is going on with each curve which allows the whole family of curves to look the way they do, making specific reference to function transformations.

2. Has some plots of some of individual curves in the family of curves to illustrate the writing they’ll be doing.

3. Has a list of things they notice about the graph and things they wonder about the graph.

At the end, I’ll photocopy the pieces onto cardstock and make a gallery in the room — but without the artist’s names displayed. I’ll give each student 5 stickers and they’ll put their stickers next to the pieces they like the most (that are not their own). I’ll invite the math department, the head of the upper school, and other faculty to do the same. The family of curves with the most stickers will win something — like a small prize, and for me to blow their artwork into a real poster that we display at the school somewhere. And hopefully the creme de la creme of these pieces can be submitted to the math-science journal that I’m starting this year.

Right now, I have a really good feeling about this. It’s low key. I can introduce it to them in half a class, and give them the rest of that class to continue working on it. I can give them a couple weeks of their own time to work on it (not using class time). And by trying to suss out the family of curves and why it looks the way it does, it forces them to think about function transformations (along with a bunch of reflections!) in a slightly deeper way. It’s not intense, and I’ll make it simple to grade and to do well on, but I think that’s the way to do it.

What’s also nice is when we get to conic sections, I can wow them by sharing that all conics are generated by $r(\theta)=\frac{k}{1+k\cos(\theta)}$. In other words, conic sections all can be generated by a single equation, and just varying the parameter $k$. Nice, huh?

PS. Since I am not going to do this for a few weeks, let me know if you have any additional ideas/thoughts to improve things!

# Recent Quadratics Stuffs from Algebra II

I am just finishing up my quadratics unit in Algebra II. We spend a lot of time on quadratics, doing everything from factoring, to completing the square, to the quadratic formula, to all sorts of graphing, the discriminant, 1D and 2D quadratic inequalities, quadratic linear systems, systems of inequalities, etc. Tons. And we didn’t even get to do the project I enjoy involving pendulums and quadratic regressions. Le sigh.

I’ve posted much of my quadratics materials before, but I thought I’d share some new/updated ones. I’m a bit exhausted, so forgive the shortness of my descriptions.

1. My Vertex Form worksheet was motivated by my frustration with students just memorizing that $y=(x-2)^2+3$ has a vertex of $(2,3)$ because you “switch the sign of the -2 and keep the 3.” Barf. (FYI: we haven’t done function transformations yet.) So I created this sheet to “guide” students to a deeper understanding of vertex form.

[.doc]

2. My Angry Birds activity was inspired by Sean Sweeney, but modified. I had taught students how to graph (by hand) quadratics of the form $y=x^2+bx+c$ and $y=-x^2+bx+c$. Students also had been exposed to the vertex form of these basic quadratics. But they hadn’t been exposed to quadratics where the coefficient in front of the $x^2$ term wasn’t “nice.” So all I did was give them four geogebra files, and had them play around. By the end of the activity, students recognized how critical the “a” coefficient was to the shape of the parabola, they started conjecturing that if you had the “a” value and the vertex and whether the parabola opens up/down that you could graph any parabola, and one pair of kids were able to convert a crazy angrybirds quadratic (with a really nasty “a”‘ value) to vertex form.

[.doc] [files]

If I’m teaching Algebra II next year, I want to ask if I can get rid of quadratic inequalities or some of the other more technical things we do, and make an entire unit/investigation on using geogebra and algebra and angrybirds to investigate quadratics.

3. My discriminant worksheet is below. It worked okay, but students still didn’t quite understand the difference between $y=ax^2+bx+c$ and $0=ax^2+bx+c$, which was the goal of the sheet. So it needs some refinement.

[

[doc]

4. Finally, below are my attempts to get students to better understand quadratic inequalities. I started with a general sheet on “visualizing function inequalities,” and then I made a guided sheet to bring more detail to things. I found out that students didn’t quite understand the meaning of the schematic diagram we drew, nor did they understand why to solve $0 we have to draw a 2D graph. Well, to be more specific, students could do the process but didn’t fully grasp why we graph $y=x^2-4x+3$. I changed up this worksheet this year, but maybe I should go back to last year’s worksheet.

[doc]

[doc]

C’est tout. With that, I’m exhausted and going to bed.

# Believe it or not… a log question which was briefly stumping us

Hi all,

A teacher approached me with the following question.

The function $\ln(x^{-2})$ has a graph that looks like:

It makes sense that the function exists for all negative x values, because when you raise a negative number to the -2 power, you’re going to get a positive number. And you can take the natural log of a positive number.

Then the teacher said to consider the following function: $-2 \ln(x)$, and the graph looks like:

Notice that you can’t input negative x values, because the domain of natural log doesn’t allow for it.

Here’s the question.

According to the log rules/properties, we know that:

$\ln(a^b)=b\ln(a)$ (obviously).

So $\ln(x^{-2})=-2\ln(x)$. But the graphs are different.

We went a little crazy trying to figure out what’s going on… For about 3 minutes, we were having a great conversation. But we quickly converged on the little text that accompanies the log rules in any textbook… and this text says that these rules work but are only valid for $a>0$.

I kinda love this as an in-class exercise (I’ll probably forget this when I get to logarithms, but maybe posting it here will prevent me from forgetting it). Because it will force kids to (a) be confuzzled, (b) talk through ideas, (c) go back to the definition and qualifications for the log rules, and (d) see that these rules are indeed valid (we didn’t break math), but they are a bit more restrictive that we might have thought.

What I love is that $\ln(x^{-2})=-2\ln(x)$ isn’t actually an identity. But we are so used to using the rules blindly, robotically, that we never think about it. But for it to be a good mathematical statement, you need to qualify it! You need to say this is only an equivalence for $x>0$. This was a good reminder for us.

# Review Activity for Rational Equations

Last year, I did a review game that I got from Sue Van Hattum. I wrote that:

[this game] forces students to ask themselves: what do I know and how confident am I in what I know? (It’s meta-cognitive like that).

I set kids up in pre-chosen pairs, and they are asked to work together. In fact, I gave kids their new seats for the quarter, so this was their introduction to their new seat partner! They then are given a booklet with problems — and each pair is asked to work only on ONE problem at a time. (For those who finish a problem before others, I have alternative problems for them to work on.) When I see almost all pairs are done, I’ll give a one minute warning… Then I ask all students to put their pencils down and pick up a pen. We go over each problem, kids correct their own work, and using the honor system, they figure out how many points they have. (Scoring below.)

You can see three sample questions from our review game below…

[The .pdf and .doc file of the 6 questions are linked.]

I explained in my last post how scoring worked…

Each group started with 100 points to wager — and they lost the points if they got the question wrong, and the gained the points if they got the question right.

Some possible game trajectories:

100 –> 150 –> 250 –> 490 etc.

100 –> 10 –> 15 –> 30 etc.

Anyway, what was great was that the game really got students engaged and talking. Each student tended to work on the problem individually, and then when they were done, they would compare with their partner.

(If you try this, you have to make sure that students know NOT to skip ahead… everyone is working on one problem at a time. Then you go over the problem, and THEN everyone starts the next problem.)

So there you go… I don’t do reviews a lot, but for rational expressions, rational equations, and circuit problems, I figured we’d need a day to tie things up. And since this is one review I think works amazingly, I figure I’d share it a second time! Thanks Sue!

# Absolute Value

So I taught absolute value equations in Algebra II. And so far I think things have gone fairly well. I read Kate Nowak’s post on how she did absolute values, and I thought I would change my more traditional introduction to them… but I didn’t. I realized that the way Kate was motivating it (with the distance on the numberline model) was great, but I felt I could still get deep conceptual understanding with the traditional way she eschewed in her post.

So I stuck with that.

I used exit cards to see how they could do… and they were okay.

But after learning how to solve $|2x-3|=5$ or $|2x-3|=-10$, I asked kids to solve things like $2-5|5x+6|=5$ or something similar. Many students said on their home enjoyment:

$2-5(5x+6)=5$ or $2-5(5x+6)=-5$.

It is unsurprising to me, and yet, it makes me want to throw up. Because what’s coming more and more into focus, and I’m sure you’re going to hear me complain about this more and more in the coming months, is how reliant students are on “coming up with rules” and “applying rules” — without thinking. They desperately want unthinking rules. And this year, because I can’t handle throwing up all the time, I’m vowing to really not give rules to them.

I really got to the heart of this “I LIKE PROCEDURES” thing with them with a true-false activity that I did, using my poor man clickers. I think this exercise highlighted how dependent my kids are on procedures and coming up with simple rules that help them in the short term… but that can hurt them in the long term… It’s a bunch of True-False questions. And when we talked about each one of them, my class saw concretely how reliant they were on misconceptions and false rules. EVERY SINGLE QUESTION led to a great short discussion.

So here they are, for you to use. Sadly, I don’t have the blank slides to share with you, because my school laptop is not with me at home now.

These were great for asking “so who wants to justify their answer?”

# School Store & Matrices

I spent a day on matrices and then we had winter vacation. Two weeks off. We came back and it took us two days to polish them off. In Algebra II, all we do is teach students some basics. I go over how to add, subtract, and multiply matrices. I remind students about multiplicative inverses. Then I introduce the identity matrix — so that we can talk about how $[A][A]^{-1}=I$. And finally we write systems of equations in matrix form, and use our calculators to solve the systems.

Early on when introducing matrices, I threw the following two slides on the board:

And then I asked, without students doing calculations, which grade took in the most money? We took a poll. Then I asked how we might figure it out. A student answered “well we take the number of sweatshirts and multiply it by the cost of each sweatshirt and add it to the…” and I said “hmmm, this sound like you’re doing a lot of multiplying and adding… we just did a lot of multiplying and adding in this funny way.” MATRICES!

So we were able to figure this out using matrices (and I showed them how to use their calculators to do this). Turns out that no student guessed the 10th grade (which was the right answer). They were so enamored by the sweatshirts that they ignored the socks! (Next year I might have them do a ranking — who made the most to who made the least.)

The next day, before we embarked on using matrices to solve systems of equations, I threw the following on the board as a do now:

FIND THE PRICES OF THE ITEMS! They just sort of sat there blankly. Well, a few said “I remember how much things cost from yesterday” but I said the school store was under a new regime of leadership and the prices have changed. I told my kids to guess and check or try anything they wanted. Most just sat there dumbfounded. We left it.

We went through class as normal, going over home enjoyment and solving systems (which is not easy to teach, btw, because you have to talk about how matrix multiplication is not commutative, how there isn’t matrix multiplication, how you need to have an inverse matrix, and how there is something called the identity matrix and how it acts like the number “1″). At the end of class I threw up the same slide.

Most kids knew what to do. They saw the system of equations, and how matrices could help them solve it.

I don’t know if I’ll keep the ordering of these problems the same — in terms of when I introduce them in class. I don’t think I gave them due deference. But for some reason, I  really enjoyed them. Although it doesn’t really answer why we do matrix multiplication the way we do it, the first day slides really show them that there is some logic to wanting to multiply and add, multiply and add…. The second day’s slide really highlights how intractable some problems might be at first glance, and how powerful matrices are to get us out of a seemingly impossible quandary.

# Square Roots and Cube Roots

I’ve posted a lot about Calculus this year, and a bit about Multivariable Calculus too. But I’m not saying too much about Algebra II. Sorry. This year something is off, and the students aren’t as successful as in years past. I’m not exactly sure what to do. I’ve asked the student-led tutoring program to lead an Algebra II study group (we’ll see if anyone signs up). I also might change my teaching practice to allow more time for students to work problems in class — because I need to see more of them working and catch their errors in thinking earlier — before they go and practice the material wrongly at home.  We’ll get through less curriculum though if I do that, and that itself is a problem, since we’ve pared down the curriculum so much.

Anyway, that’s generally where I am with Algebra II.

Specifically, I just wanted advice on how you guys teach cube roots (and fourth roots and fifth roots, etc.).

My ordering usually goes:

1. Turn to your partner, and explain to them what $\sqrt{5}$ means to someone who doesn’t know anything about square roots.

Students generally say that it’s the number that when multiplied by itself will give you 5. I then say “if the person doesn’t know anything about square roots, you might want to give them an easier example, like $\sqrt{25}$… and explain how that is 5. But that $\sqrt{5}$ isn’t a perfect square so you’d get some number between 2 and 3. Yadda Yadda. I also then talk about the geometric interpretation (the side length of a square with area 5). Then I go back to the “it’s the number that when multiplied by itself will give you 5.”

I do not talk about there being two answers to “the number when multiplied by itself gives you 5″ and the principal square root business. Because I want to use this to capitalize on their understanding of cube roots.

2. Then I put up $\sqrt[3]{8}$ and say this is 2. And to think about what this funny $\sqrt[3]{}$ symbol means. They get it. I put up a bunch more, and they usually can solve them. I put some negatives under the cube root symbol too.

3. I then ask them what $\sqrt[3]{}$ means, and they say “the number that when multiplied by itself three times gives you the value under the cubed root sign.”

4. I then throw up a bunch of problems, and three of these include $\sqrt{49}$ and $\sqrt{-49}$ and $-\sqrt{49}$.

This is where the trouble comes in.

Some students now say $\sqrt{49}$ is $\pm 7$. Because 7 and -7 are numbers when multiplied by itself which equals 49.

Here’s where I use the whole: “Don’t lose what you already know! Would you say $\sqrt{49}$ is -7 ten minutes ago? No. You’re right, that there are two numbers which, when multiplied by themselves, give you 49. So we can tell them apart, we say $\sqrt{49}$ is the positive one and $-\sqrt{49}$ is the negative one. So don’t lose what you know. When you see a radical sign, it just represents a single number. If there’s a negative in front of it, it represents a negative number. If not, it’s a positive number. Just like what you’ve always known.”

Okay, now I know the idea of “principal square roots” and all that. And I honestly don’t want to have this whole discussion about principal square roots with them, because every time I do, they come out more confused.

So here’s my question.

How do you introduce cube (and higher) roots? How do you engage with this idea of principle square roots so that students don’t leave confused? I just can’t get it totally right.

And just so I am being clear, I know the properties of square roots and cube roots and all that. I’m not looking for someone to explain that to me. I want a way to teach my KIDS these without confusing them all up. And I bet crowdsourcing is a good way to get ideas for next year.

# Two Worksheets

ONE

On Thursday, I’m going to be introducing absolute value inequalities. Last year I used the picture below as motivation.

I then tried to work backwards to show kids absolute value inequalities. It wasn’t too hot a success. Certainly the “application” wasn’t a motivator, and working backwards just confused things.

This year, I’ve decided to start with a warm up. Without them knowing anything, I’m going to ask them to do this for the first 7 minutes of class with their partners.

I already can see the great questioning and discussion that this simple worksheet will generate between partners. And then, when we come together: WHAM! powerful! It’s a simple thing, but Oh! So! Delicious!

After that, after we see some patterns and make some conclusions… then, then I can throw up the picture of the bag, and talk about it meaningfully. And have kids work backwards from their own conclusions to finding a way to express that region mathematically, using absolute value inequalities.

TWO

I’m introducing limits tomorrow. I pretty much have carte blanche in what I do. Last year what I did was sad. Like SAD. Like: “Here’s what a limit is. Get it?” This year, I’m stealing pretty much from CalcDave wholesale. Here’s his calculus questionairre. And here’s what I made.

Pretty much the same thing. Then I’d like to somehow have them start thinking about how to get velocity from a position versus time graph. Haven’t quite figured that out yet. Either that, or Zeno.

# Histograms, Standard Deviations, and Digital Cameras

Our last unit in Algebra II was statistics — and it was a hurried unit. (As last units always are.)

One of the topics I was covering was histogram basics. And I wanted to make it somewhat interesting. So I went online, and came across a page which explained how to understand histograms that your digital camera produces. You know what I’m talking about, right?

That’s the one. How do you get it on your camera? Heck if I know. I just pushed a lot of buttons and eventually the histogram appeared.

Because I had about 20 minutes, I just lectured my kids on how this histogram worked.The histogram has 256 columns (numbers 0 to 255). Each pixel on your camera is assigned a number from 0 (representing pure darkness) to 255 (representing pure lightness). Then the height of each bar represents the number of pixels with that particular level of darkness/lightness.

By that one little piece of information, you can start telling a lot about a photo. Such as when it is over-exposed and under-exposed, and when there is too much or too little contrast. You might wonder how photo editing software can increase the contrast or correct for a photo being over/under-exposed. One you learn about this, the answer is pretty simple. The program reassigns each pixel with a different brightness.

See examples that I cribbed from the website on my smartboard. Pay special attention to how the over-under exposed histograms differ from the “ideal” histogram (and similarly for the too high/too little contrast):

I really enjoyed learning about this, and sharing what I learned with my students. But next year, I want to do something more. I want students to take photos and play with them in some image editing software — and see what happens to the data as they modify the image in certain ways. What does brightness mean? Will things change if the image goes from color to black and white? What does sharpening the image do to the histogram? I want them to talk about mean, median, and mode — and how they change. I want them to talk about standard deviation — and how it changes. I want them to talk about range and shape — and how they change. I want them to make a short writeup explaining their findings.

Look at what Picasa (free) offers:

You get the histogram (bottom left)! You also get all these ways to modify the picture!

And the histogram changes as you modify it! In REAL TIME as you slide sliders!

I don’t know quite yet how to make this rigorous or ways to ensure they’re learning. It’s kinda bad, because I just want to play around with this and discover what all these things do myself, not knowing what I want them to get out of it. I just want to explore. I’m not thinking backwards. But I suspect a good short bit on the shape of data can be made from this. (Alternative reading: I wouldn’t begrudge any of you if you, say, went out and made a short unit based on this and sent it to me.)