I am a high school math teacher in Brooklyn, New York. I enjoy getting students excited about math by being math’s loudest and most passionate cheerleader.

This is going to be a super short blogpost. But I’m excited about a visualization I came up with today — as I was working on a lesson — for showing why Pascal’s Triangle works the way it does with binomial expansions.

I’m sure that someone has come up with this visualization before. It feels so obvious to me now. That that didn’t make me any less excited about coming up with it! I immediately showed it to two other teachers because I was so enthralled by it. #GEEKOUT

I am thinking how powerful a gif this would be. Start out with 1. Have two arrows emanate from that 1 (one arrow saying times x and one arrow saying times y) and then it generates the next row: 1x 1y. And again, two arrows emanate out of both 1x and the 1y (arrows saying times x and times y). And generating 1x^2 1xy 1xy 1y^2. Then then a “bloop” noise as the like terms combine so we see 1x^2 2xy 1y^2.

And this continues for 5 or so rows, as this sinks in.

Then at the very end, some light wind chime twinkling music comes up and all the variables disappear (while the coefficients stay the same).

Of course good color choices have to be made.

Who’s up for the challenge?

Okay, I’m guessing something similar to this already exists. So feel free to just pass that along to me. Now feel free to go back to your regularly scheduled program.

A few days I got an email from someone (Jeremy Jones) who wanted me to look at their video on standard deviation. And then today, I was working with Mattie Baker at a coffeeshop. He was thinking about exactly the same thing — how to get standard deviation to make some sort of conceptual sense to his kids. He said they get that it’s a measure of spread, but he was wondering how to get them to see how it differs from the range of a data set (which also is a measure of spread).

Of course I was hitting a wall with my own work, so I started thinking about this. While watching Jeremy Jones’s video, I started thinking of what was happening graphically/visually with standard deviation.And I had an insight I never really had before.

So I made an applet to show others this insight! I link to the applet below, but first, the idea…

Let’s say we had the numbers 6, 7, 7, 7, 11. What is the standard deviation?

First I calculate the mean and plot/graph all five numbers. Then I create “squares” from the numbers to the mean:

The area of those squares is a visual representation of how far each point is from the mean.[1] So the total areas of all those five rainbow squares is a measure of how far the entire data set is from the mean.

Let’s add the area of all those squares together to create a massive square.

As I said, this total area is a measure of how far the entire data set is from the mean. How spread out the data is from the mean.

Now we are going to equalize this. We’re going to create five equal smaller squares which have an area that matches the big square.

We’re, in essence, “equalizing” the five rainbow colored squares so they are all equal. The side length of one of these small, blue, equal squares is the standard deviation of the data set. So instead of having five small rainbow colored squares with different measures from the mean, the five equal blue squares are like the average square distance from the mean. Instead of having five different numbers to represent how spread out the data is from the mean, this equalizing process lets us have a single average number. That’s the standard deviation.

I’m not totally clear on everything, but this visualization and typing this out has really help me grok standard deviation better than I had before.

I created a geogebra applet. You can either drag the red points up and down (for the five points in the data set), or manually enter the five numbers.

{4, 4, 4, 4, 4}. Make a prediction for what the standard deviation will be. Then set the five numbers and look at what you see. What is the standard deviation? Were you right?

{8, 8, 8, 8, 8}. Make a prediction for what the standard deviation will be. Then set the five numbers and look at what you see. What is the standard deviation? Were you right?

Set the five numbers to {2, 4, 4, 4, 6} and look at what you see. What is the standard deviation?

Consider the number {5, 7, 7, 7, 9}. Make a prediction if the standard deviation will be higher or lower or the same as the standard deviation in #3. Then set the five numbers to {5, 7, 7, 7, 9} and look what you see. What is the standard deviation? Were you right?

Consider the numbers {3, 7, 7, 7, 11}. Make a prediction if the standard deviation will be higher or lower or the same as the standard deviation in #4. Explain your thinking. Then set the five numbers to {3, 7, 7, 7, 11} and look at what you see. What is the standard deviation? Were you right?

Consider the numbers {3, 6, 7, 8, 11}. Make a prediction if the standard deviation will be higher or lower or the same as the standard deviation in #5. Explain your thinking. Then set the five numbers to {3, 6, 7, 8, 11} and look at what you see. What is the standard deviation? Were you right?

What do you think the standard deviation of {4, 8, 8, 8, 12} be? Why? Check your answer with the applet.

Can you come up with a different data set which matches the standard deviation in #6? Explain how you know it will work.

Set the five numbers to {4, 4, 4, 4, 4}. Initially there are no squares visible. The standard deviation is 0. Now drag one of the numbers (red dots in the applet) up. Describe what the squares look like when they appear? Eventually drag that number to 15. What do you notice about the standard deviation? Use your understanding of what happened to describe how a single outlier in a data set can affect the standard deviation

Okay, I literally just whipped the applet up in 35 minutes, and only spent the last 15 minutes coming up with these scaffolded questions. I’m sure it could be better. But I enjoyed thinking through this! It has helped me get a geometric/visual sense of standard deviation.

Now time to eat dinner!!!

Update: a few people have pointed out that the n in the denominator of the standard deviation formula should be n-1. However that would be for the standard deviation formula if you’re taking a sample of a population. This post is if you have an entire population and you’re figuring out the standard deviation for it.

[1] One might ask why square the distance to the mean, instead of taking the straight up distance to the mean (so the absolute value of each number minus the mean). The answer gets a bit involved I think, but the short answer to my understanding is: the square function is “nice” and easy to work with, while an absolute value function is “not nice” because of the cusp.

This is a short continuation of the last blogpost.

In Advanced Precalculus, I start the year with kids working on a packet with a bunch of combinatorics/counting problems. There is no teaching. The kids discuss. You can hear me asking why a lot. Kids have procedures down, and they have intuition, but they can’t explain why they’re doing what they’re doing. For example, in the following questions…

…students pretty quickly write (4)(3)=12 and (4)(3)(5)=60 for the answers. But they just sort of know to multiply. And great conversations, and multiple visual representations pop up, when kids are asked “why multiply? why not add? why not do something else? convince me multiplication works.”

Now, similar to my standard Precalculus class (blogged in Nominations, Part I, inspired by Kathryn Belmonte), I had my kids critique each others’s writings. And I collected a writeup they did and gave them feedback.

But what I want to share today is a different way to use the “Nomination” structure. Last night I had kids work on the following question:

Today I had kids in a group exchange their notebooks clockwise. They read someone else’s explanations. They didn’t return the notebooks. Instead, I threw this slide up:

I was nervous. Would anyone want to give a shoutout to someone else’s work? Was this going to be a failed experiment? Instead, it was awesome. About a third of the class’s hands went in the air. These people wanted to share someone else’s work they found commendable. And so I threw four different writeups under the document projector, and had the nominator explain what they appreciated about the writeup. As we were talking through the problem, we saw similarities and differences in the solutions. And there were a-ha moments! I thought it was pretty awesome.

(Thought: I need to get candy for the classroom, and give some to the nominator and nominee!)

The best part — something Kathryn Belmonte noted when presenting this idea to math teachers — is that kids now see what makes a good writeup, and what their colleagues are doing. Their colleagues are setting the bar.

***

I also wanted to quickly share one of my favorite combinatorics problems, because of all the great discussion it promotes. Especially with someone I did this year. This is a problem kids get before learning about combinations and permutations.

Working in groups, almost all finish part (a). The different approaches kids take, and different ways they represent/codify/record information in part (a), is great fodder for discussion. Almost inevitably, kids work on part (b). They think they get the right answer. And then I shoot them down and have them continue to think.

This year was no different.

But I did do something slightly different this year, after each group attempted part (b). I gave them three wrong solutions to part (b).

And it was awesome. Kids weren’t allowed to say “you’re wrong, let me show you know to do it.” The whole goal was to really take the different wrong approaches on their own terms. And though many students immediately saw the error in part (a), many struggled to find the errors in (b) and (c) and I loved watching them grapple and come through victorious.

At TMC this past summer, Kathryn Belmonte introduced an idea about sharing student work in the classroom. Something she termed “NOMINATIONS!” I loved the idea — and wanted to use it when kids do their explore-math project. But I saw it was so flexible, and pretty early on, the time was right to test it out. So I modified it slightly and this post is about that…

In all of my precalculus classes (I teach two standard sections and one advanced section), my kids are being asked to do tons of writing. A few who have had me before in geometry are used to this, but most are not. And honestly: getting down what mathematical writing is, and how to express ideas clearly, is hard.

So what do I do? I throw them into the deep end.

On day two of class, I ask them to write an answer to a problem for a seventh grader to understand. On the third day of class, they come in, and are given the name of the student who comes after them alphabetically (and the last person is given the name of the first person alphabetically). Then they read these instructions:

Everyone moves to the desk of the name they were given. Then I project on the board:

And I give students to read through a different student’s solution. They have to make sense of it — pretending to be new to the problem. And then they critique it. Eventually, probably after 3-5 minutes, I left them return to their seats. They read over the comments. I talk about why the feedback is important. And how specific feedback is useful (so “good explanation” is less useful than “your explanation of how the groups were made was easy for me to follow”). And then we continue on with class.

Here are examples of some post-its (front of a few, then back of a few):

To follow up: that night for nightly work, I gave students a writing problem — a simple probability problem. My hope was that this would help them pay attention to their explanations. I collected the problem and read through the writeups.

They weren’t so hot. Most of them didn’t talk about why and some didn’t have any diagrams or visuals to show what was happening with the problem. So I marked them up with my comments. (They got full credit for doing it.) The next day I handed them back and shared my thoughts. I also shared a copy of a solid writeup — one that I had created — along with four or five different possible visuals they could have used. (I realized –after talking with Mattie Baker about this — that I couldn’t really get my kids from point A to point B unless they saw what point B looked like, and what my expectations were.)

At this point, I wanted to figure out if they were taking anything away from all of this. So I created a page with three questions. A formative assessment for me to see what my kids understand and what they don’t about the content. But I also asked them to take all the feedback they’ve gotten about writing and explanations, and explain the heck out of these problems. Here’s an example of one of the problems (one I’m particularly proud of):

I collected them today. I haven’t looked through them carefully yet, but from a cursory glance, I saw some thoughtful and extensive writeups. And even from this cursory glance, I can see that these two activities — plus all the conversations we’re having about explaining our thinking in class — have already made an impact.

Yes, they’ve gotten some ideas of what a good writeup looks like. They know diagrams can be helpful. They know words to explain diagrams are important. They know the answer to why is what I’m constantly looking for when reading the explanations.

But more important to me is the implicit message I’m trying to send about my values in the classroom. I think a lot about implicit messaging to communicate my values, especially at the start of the year. And I am confident my kids know with certainty that I value all of us articulating our thinking as best as we can, both when speaking but also when doing written explanations.

Today was my first day with kids. I can’t tell you how terrified I was to be back. I had about a zillion normal reasons (the standards: do i still remember to teach? so many kids names to learn and i’m terrible at it! what if I totally suck?). I also have a lot on my plate right now, a few of which are out of the ordinary, which have put me in a weird headspace. #cryptic #sorry

However I had a really good day today. I saw my advisory and two of my four classes. I even went to some of the varsity volleyball game after school!

This post isn’t about my kids or my classes. It’s going to be about some things I’ve done at the start of this year.

(1)Inclusivity. I read a book about trans teens this summer. We had a lot of conversations about pronouns last year. We as a school have taken gendered pronouns out of our mission statement. Last year I included this in my course expectations:

But this year, in my get-to-know-you google form that I give to kids, I asked for their pronouns.

Chances are, I probably am not going to get any different answers that what I expect this year. But I’m not including this question for the majority of kids. I want to be ready when that first kid gives me pronouns that differ from what I may expect. I want that kid to know they can find comfort (not just safety) in our room. And I want all kids to know things that I value. And I think this question sends that message — no matter who the kid is.

That’s the idea behind it. Who knows if my intention is how the kids will understand/interpret it?

(2)Mattie Baker and I were working at a coffeeshop before the year started, and he showed me his class webpage, which had this video (which I’d seen before) on it front and center:

I loved how *real* this video felt to me. Not like something education schmaltzy which makes me want to roll my eyes. I then went searching for a twin video that explicitly talks about the growth mindset. I had a dickens of a time finding one that I felt would be good for students to watch, but didn’t seem… well… lame. I found one:

So as part of the first set of nightly work, I’m having kids watch these videos and write a comment on them in google classroom. (So others can read their comments.) As of writing this, one class has already had two kids post their comments (even though I don’t see them until next week). I read them and my heart started singing with happiness. I have to share them:

Two videos aren’t a cure-all. But having kids realize how important having a positive can-do attitude, and how important it is to look at math as a skill to be developed (rather than something you’re innately born with and is fixed) is so important to me. I have to remember to be cognizant about how important this stuff is, and how important it is to reinforce daily.

(3)In both of my classes today, one student said something akin to “I first thought this, but then I talked with Stu (or listened to the whole class discussion) and I changed my mind.” I stopped both classes and made a big deal about how important that was for me. And how those types of statements make my heart sing. And why they make my heart sing. So they should say those sort of things aloud a lot. Okay, so I said it once in each class. How can I remember to say it a lot more? In any case, it was a teacher move I was proud of.

Oh oh another teacher move… I saw when one student was sharing their thinking with the class, but not everyone was facing the student. And I remembered Mattie Baker and Chris Luzniak’s training from this summer (on dialogue in the classroom). I told everyone they had to face the person that was speaking. I need to remember next week to make this more explicit — and talk about (or have kids articulate) what they should look like when actively listening to someone. And why it’s important to give this respect to someone. They are sharing their thinking — which is a piece of them — with us. They took a risk. We need to celebrate that. And try to learn from their thoughts. Doing anything else would be a disservice to them and to our class. (Okay, clearly you can tell I’m thinking through this in real-time right now by typing.)

(4) Robert Kaplinsky has created a movement around opening classrooms up. I personally hate being observed. Before someone comes, I freak out. Of course as soon as I start teaching, I absolutely forget that they are watching. Totally don’t even recognize them as an entity. In fact, I think I often teach better, probably because I’m subconsciously aware I’m being watched so I’m hyperaware of everything I’m doing. But leading up to it is horrible. And I also hate the idea of “surprise visits” because… well, who likes them?

That being said, I know that getting feedback is important, and I know that in my ideal school, classrooms wouldn’t be silos. So I joined in. Not for all my classes… I need to dip my toe in gently. But I posted this next to my classroom door:

Next week or the week after, I’ll probably put this up as a “do now” and ask kids “what do they notice/wonder?” about it. Then I’ll tie it into a conversation about growth mindset and the videos they watched.

(5)For the past two years, I’ve been teaching only advanced courses. (In fact, because of that, I asked to teach a standard course… I have taught many, and it was weird to not have that on my plate for two years.) And I heard from someone that a few kids were nervous about having Mr. Shah because “he teaches the really hard courses? will he be able to teach us?”

I know that my first few classes with these kids need to show them that I am different than they expected. I also was proud of this paragraph I put on the first page of their course expectations…

(6) I met my advisory for the first time. Seniors. The thing is: we’re ramping up our advisory program to be more meaningful. Advisors are going to be with their advisees for four years. We are going to be the initial point of contact for many things. And we want to be there to support and celebrate our advisees in a way that we haven’t been able to in our previous set up.

But for all this to happen, I need to form relationships with my advisees. Relationships that go beyond pleasantries. In our training for our new advisories (amazing training… I think I should write a post to archive that thinking before I forget it… done by the Stanley King Institute) we did an exercise. We found someone we didn’t know (I found a new teacher at our school). We had to think about something meaningful to us, and something real (not something like our favorite sports teams… sorry sports fans)… and then talk about it with our partner for EIGHT MINUTES. Anyone who has been a teacher knows that speaking about anything for eight minutes straight is tough. It feels like eternity. While that person is speaking, the other person is actively listening. They can say a few words here and there, like “oh yeah…” or “totally,” but it wasn’t about having a conversation.

Normally I’d roll my eyes at something like this. But at the end, I felt like I got to know this new person at our school pretty well… actually, considering we only had eight minutes, amazingly well… and we bypassed all the initial superficial stuffs. That stuff, like movies and books and stuff, we’ll get to later. Yes, it was awkward. But yes, it worked.

So here’s how I adopted it for my advisory. I met with them today to do a bunch of logistics, and then I took them to a different room. I had cookies, goldfish, crackers, and a cold drink for them. And I explained this exercise. And I said: “I want to do this with you. I want to get to know you.” And so I took out the notecards I prepared, and I shared stuff about my life with them. And they were rapt. I told them about stuff going on in my family that was exciting and stuff going on that was tough, I told them “things I wish my students knew” (this is such a great way to flip “things I wish my teacher knew”). I told them my total anxiety for the start of the school year and why I had it, and I told them my total excitement for the start of the school year and why I had it. I even said: “I never feel like I’m a good enough teacher.” When I was saying that, I wondered how many kids think “I never feel like I’m good enough.”

A photo of my index cards are here… but I only used them as launching points. I didn’t want to be rehearsed.

I’m a guarded person, and I made sure never to cross the line between personal and professional, but when I finished, I sensed some (all?) of them were processing that a teacher opened up to them in this way. A few thanked me for sharing with them.

I wanted to set up an initial connection, and send the message I want you to know that I’m not an advisor in name only… I’m opening up to you because I want you to believe that when you’re ready, you can open up to me. They’re seniors. They have a lot figured out. But I hope they know I’m here for the stuff they don’t have figured out.

In the next week and a half, I have 10 minute meetings with all of my advisees individually. I told my kids they are going to talk to me about what’s meaningful to them for 8 minutes. I acknowledged it would feel awkward. I told them they didn’t need to open up in any way that made them feel uncomfortable. But I wanted them to speak about whatever is meaningful to them. We’ll do favorite books later. Now I’ll get to know them on a more personal level. [1]

(6)As you might have noticed from #4 above, I’m trying to be better about formative assessments. I want to make sure I know what kids are thinking, and where they are at, and use that to refine or alter future classes. I haven’t tried this out yet (today was just our first day!!! I only saw two classes!!!), but I made a google form for exit tickets.

This is a #MTBoS sample version, so feel free to click on it, and fill out fake feedback to get an idea of the form.

Pretty awesome idea, right? I didn’t want to have a bunch of pre-printed slips (something I knew I wouldn’t actually do).

(7)I took a page from Sara Van DerWarf’s playbook. I didn’t do this on the back of name tents, but I have a separate sheet that they’re filling out. For my two classes today, I asked them to share something about themselves that would help me get to know them as something other than a kid in our math class. Some kids gave a lot, some a little, but I learned something about each one of my kids. As I’ve mentioned, I’m terrible with names. But what’s nice is on this sheet I created, I put photos (they’re in a school database for us to use) and knowing something about them is helping me remember their names. It’s odd and unexpected and lovely. Kids interested in arts/photography/social justice/sports/debating-arguing/nature/etc. I liked writing that little note back to my kids. I don’t know what question I’ll ask next. I may ask them “Math is like…” (like James did). For the penultimate one, I should definitely take a cue from Sara and ask them to ask ME a question.

I have a few more ideas for posts percolating. I hope that I get the time and motivation to write them. But it’s nice to be back!!! SCHOOL IS IN SESSION!

***

[1] This is what I included in my email to my advisees:

I know it may feel awkward, but when you meet with me during this meeting, you’re going to speak for 8 minutes about things that are meaningful to you. So something more than a listing of your favorite books/movies. If you need help thinking about this: what makes you tick? what makes you gasp? what are your thoughts about senior year and the future? what could you not imagine doing? what are you feeling? what keeps you up at night? These are all questions that might help you find things that are truly meaningful to you. I found it really helpful to have an index card of things when I was talking with you, because I was nervous. I suggest doing that!

CAVEAT: There isn’t any deep math in this post. There aren’t any lessons or lesson ideas. I was just playing with quadratics today and below includes some of my play.

I’ve been struggling with coming up with a precalculus unit on polynomials that makes some sort of coherent sense. You see, what’s fascinating about precalculus polynomials is that to get at the fundamental theorem of blahblahblah (every nth degree polynomial has n roots, as long as you count nonreal roots as well as double/triple/etc. roots), one needs to start allowing inputs to be non-real numbers. To me, this means that we can always break up a polynomial into n factors — even if some of those factors are non-real. This took up many hours, and hopefully I’ll post about some of how I’m getting at this idea in an organic way… If I can figure that way out…

However more recently in my play, I had a nice realization.

In precalculus, I want students to realize that all quadratics are factorable — as long as you are allowed to factor them over complex numbers instead of integers. (What this means is that is allowed, is allowed, but so are and and . (And for reasons students will discover, things like won’t work — at least not for our definition of polynomials which has real coefficients.)

So here’s the realization… As I started playing with this, I realized that if a student has any parabola written in vertex form, they can simply use a sum or difference of squares to put it in factored form in one step. I know this isn’t deep. Algebraically it’s trivial. But it’s something I never really recognized until I allowed myself to play.

I mean, it’s possibly (probable, even) that when I taught Algebra II ages ago, I saw this. But I definitely forgot this, because I got such a wonderful a ha moment when I saw this!

And seeing this, since students know that all quadratics can be written in vertex form, they can see how they can quickly go from vertex form to factored form.

***

Another observation I had… assuming student will have previously figured out why non-real roots to quadratics must come in pairs (if p+qi is a root, so is p-qi): We can use the box/area method to find the factoring for any not-nice quadratic.

And we can see at the bottom that regardless of which value of b you choose, you get the same factoring.

I wasn’t sure if this would also work if the roots of the quadratic were real… I suspected it would because I didn’t violate any laws of math when I did the work above. But I had to see it for myself:

As soon as I started doing the math, I saw what beautiful thing was going to happen. Our value for b was going to be imaginary! Which made a+bi a real value. So lovely. So so so lovely.

***

Finally, I wanted to see what the connection between the algebraic work when completing the square and the visual work with the area model. It turns out to be quite nice. The “square” part turns out to be associated with the real part of the roots, and the remaining part is the square associated with the imaginary part of the roots.

***

Will any of this make it’s way into my unit on polynomials? I have no idea. I’m doubtful much of it will. But it still surprises me how I can be amused by something I think I understand well.

Very rarely, I get asked how I come up with ideas for my worksheets. It’s a tough thing to answer — a process I should probably pay attention to. But one thing I know is part of my process for some of them: just playing around. Even with objects that are the most familiar to you. I love asking myself questions. For example, today I wondered if there was a way to factor any quadratic without using completing the square explicitly or the quadratic formula. That came in the middle of me trying to figure out how I can get students who have an understanding of quadratics from Algebra II to get a deeper understanding of quadratics in Precalculus. Which meant I was thinking a lot about imaginary numbers.

Megan Schmidt is obsessed with spirals. Her obsession got me hooked — for hours — on a math problem. I thought it would take maybe an hour or two, but I’m still at it and I’ve probably been working four or five hours.

I’ve been having so much fun with it.

Here’s the problem. Look at the spiral below…

We see that 1 is located at (0,0).
We see that 2 is located at (0,1).
We see that 8 is located at (-1,0).

If we continue this spiral in this manner, can you come up with a formula for the coordinates of the kth number?

So what I want to know is if we consider the number 2016, can we come up with a way to precisely define where it is? What about 820526487?

One easy way around this is to write a computer program that just brute forces our way through it. So here’s the constraint: I want a closed formula for the x-coordinate and y-coordinate. That means no recursion! No if/then statements! Just an equation that relies on k only.

You know one of the most frustrating things? Going down a path and feeling good about it, even though it is pretty complicated. And then having a new insight on how to attack the problem (which *just* happened to me now as I typed up the problem and look at the image I created for this post) [1]. And realizing that approach might yield it’s secrets so much easier!

In any case, I thought I’d share the problem because it’s given me so much enjoyment thus far. If you do get obsessed and solve it, please feel free to put your answer in the comments. I have a feeling there are a variety of valid solutions which look very different but yield the same answer.

[1] What this reminds me is how slight changes in representations can lead to new insights! Before I was using this image that Megan sent me:

UPDATE!: I solved it!

If you want to see that I did solve it, check out this Geogebra sheet. It won’t give away how I solved it (unless you download it, look at how I defined each cell, and then reverse engineered it).

So yeah… 2016 is at (-22,13), and 820526487 is at (-14322, 4784).

I am so proud of myself! I came up with a closed form solution!!!

I am going to put a “jump” below here, and then show what my solution is, and write a little about it. So only read below the jump (meaning: after this) if you want some spoilers.