My older son is working through Art of Problem Solving’s Precalculus book this year. I love this book and am happy to help him work through it. For now the topic is trig functions. Here are three nice conversations we’ve had in the last week:

(1) Exploring some parametric plots:

We had been looking at graphs of sin(x) and cos(x), but what if I made the x-coordinate a function cos(t) and the y-coordinate function of sin(t)? What would that look like?

(2) A clever homework problem that gave him some trouble

The question to asked him to find the product:

(3) Talking about the sum and difference formulas

This morning he was working through the geometric proofs in the book that give you the sum and difference formulas for cos(a + b) and sin(a + b). After that I showed him how the formulas come from the formula:

I’ve never taught a precalculus class before, so I’m explaining most of these ideas for the first time. Hopefully the book will give him a good foundation and my fumbling around will show him one or two fun ideas. I think this will be a fun year.

At dinner last night the topic of sending humans to Mars came up. I don’t remember how or why – we just started talking about it and my younger son was really interested. When I asked the boys if there was anything they wanted to talk about for our project this morning the topic was Mars.

The tiny little problem with this topic is that I know absolutely nothing about it! But with a little prep work this morning I was able to throw together what was hopefully a fun 30 min.

We started by watching this short video:

After the video we talked about a few of the ideas. I almost went down the rabbit hole of explaining a bit of mechanics, but probably luckily changed my mind right before jumping in to that topic.

The next topic was Kepler’s 3rd Law. This Law says (oversimplifying) that for planets orbiting the Sun, the time of the orbit squared divided by the radius cubed is constant.

I introduced the law and did the computations for Earth and Mars.

Finally, since my older son is just starting to study trig, I wanted to show a really neat curve. I forget where I saw this idea previously, but it is a fun picture and also a neat trig example.

The question is, if we plot the midpoint of the line connecting the Earth and Mars as they orbit the sun, what does the curve look like?

This was a fun project and is also definitely a neat trig example. For a great history lesson of how humans learned more about Mars check out Terry Tao’s incredible public lecture – “The Cosmic Distance Ladder” (he’s talking about Mars around 58:00 – though maybe earlier, too):

Numberphile put out a great video on the so-called “pancake number” – see the 3rd video below. I watched it when it came out and thought it would be a fun project for the kids to explore.

Unfortunately I got mixed up on the procedure when we were doing the first two parts of the project this morning. So, at the beginning of the project we weren’t actually studying the “pancake numbers” but rather a variant. Not a big deal, though it required an explanation after we watched the video!

We started with an introduction to the sorting problem and studying the case of trying to put 5 elements in order using (what I thought were) the pancake sorting rules.

Next I asked the kids to explore the problem in general and they looked carefully at a few simple cases. One neat thing here is that one relationship between sorting N and N+1 blocks that they thought would be true turned out not to be true.

Now we move on to view the Numberphile video. Katie Steckles does such a great job here (and I love that the sheet of paper from the Graham’s number video is up on the wall behind her!).

To wrap up we returned to the white board and played around with the correct rules. It was nice that they were able to find an example with 3 blocks that had a different result when we applied the right rules.

Definitely a great video by Numberphile and a fun project for kids. It is always really fun to play with an unsolved problem that is accessible to kids.

Today I decided to revisit that project. We started by looking at the same idea from algebra:

Does ?

At first we talked about the two equations using ideas from algebra and arithmetic.

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Now I asked the boys for their geometric intuition and then showed them the 3d printed graphs of the two functions.

This part ran a little long while my younger son was stuck on a small but important point about the graph – I didn’t want to tell him the answer and it took a couple of minutes for him to work through the idea in his mind.

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Next I showed them 3d prints of and and asked them to tell me which one was which. It is really neat to hear the reasoning that kids use to go from shapes to equations.

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For the last part of the project I asked the boys to come up with their own algebra “mistakes” for us to explore. My older son chose to compare the graphs of and .

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My younger son chose the two equations and . Changing the + to a – in our first set of equations turns out to have some pretty interesting geometric consequences – “it looks sort of like a saddle” was a fun comment.

One especially interesting idea here was exploring where . We used Mathematica’s ContourPlot[] function to explore those two lines because those lines weren’t immediately obvious on the saddle.

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I’m happy to have had the opportunity to revisit this old project. I think exploring simple algebraic expressions is a fun and sort of unexpected application of 3d printing.

My older son is starting to learn trig and I wanted to show him the surprising connection between the exponential function and trig functions. Grant Sanderson’s video on Pythagorean triples from earlier this year came to mind:

After my son watched the video tonight I asked him to talk about some of the ideas. Here’s what he had to say – I was happy that many of the ideas that Sanderson presented sank in:

Now I showed him the incredible formula = Cos(x) + i*Sin(x). Although I just present the formula, the complex number ideas after that are exactly the same ideas that are in Sanderson’s video:

Now I showed him how this formula makes some of the standard trig identities really easy to both derive and remember. The ideas here were really the inspiration for the video because he asked me this morning if the trig functions were linear.

We specifically looked at formulas for Cos(2x) and Sin(2x) here.

Now we used the formulas in a couple of simple examples. First we looked at 45 degrees, then 30 degrees, and then we looked at the angles in a 3-4-5 triangle:

This project was a fun way to introduce an amazing idea in math and also hopefully a nice way to show my son that ideas in trig go way beyond triangles.

My older son has just started the trigonometry this week. I know the topic can be a little dry at the beginning, so I wanted to show him more than just unit circle exercises.

Today we looked at a few fun curves that you can make just by playing around with trig functions.

I stared by showing some simple graphs and then we moved to some 3d shapes:

Next we took a cue from an old project inspired by Henry Segerman:

Hopefully both the shape and the shadows come through in the filming – I haven’t figured out how to shoot shadows very well yet.

Finally I let the kids play around with the Mathematica code for a bit to create their own shapes. They had a couple of pretty fun ideas.

There was one little issue that came up on my younger son’s plot, unfortunately. I didn’t have enough detail in the plot to multiply the range by 10. That’s why his picture fuzzed out quite a bit. I didn’t see the problem on the fly, though, and wasn’t able to fix it in real time.

I’m excited to help my son learn about trig. Hopefully a few projects like this one will help him see that that there’s more to trig than just triangles!

During the week I attending a neat talk at Harvard given by Tim Gowers. The talk was about a intransitive dice. Not all of the details in the talk are accessible to kids, but many of the ideas are. After the talk I wrote down some ideas to share and sort of a sketch of a project:

We started the project today by reviewing some basic ideas about intransitive dice. After that I explaine some of the conditions that Gowers imposed on the dice to make the ideas about intransitive dice a little easier to study:

The next thing we talked about was 4-sided dice. There are five 4-sided dice meeting Gowers’s criteria. I thought that a good initial project for kids would be finding these 5 dice.

Now that we had the five 4-sided dice, I had the kids choose some of the dice and see which one would win against the other one. This was an accessible exercise, too. Slightly unluckily they chose dice that tied each other, but it was still good to go through the task.

Now we moved to the computer. I wrote some simple code to study 4-sided through 9-sided dice. Here we looked at the 4-sided dice. Although it took a moment for the kids to understand the output of the code, once they did they began to notice a few patterns and had some new ideas about what was going on.

Having understood more what was going on with 4-sided dice, we moved on to looking at 6-sided dice. Here we began to see that it is actually pretty hard to guess ahead of time which dice are going to perform well.

Finally we looked at the output of the program for the 9-sided dice. It is pretty neat to see the distribution of outcomes.

There are definitely ideas about nontransitive dice that are accessible to kids. I would love to spend more time thinking through some of the ideas here and find more ways for kids to explore them.