## Triangle and construction review

My younger son has been studying Art of Problem Solving’s Introduction to Geometry book this year. He’s been doing most of the work on his own. Every now and then, though, I decided to check in and see how things are going. Today the topic he was looking at when I checked in was constructions. The two questions on the table when I stopped by were:

(i) Construct the incircle of a triangle, and

(ii) Construct the circumcircle of a triangle

Here’s how things went.

(i) The incircle

He has the basic idea for how the construction works, but misses one important idea. That idea is that the tangents of the incircle are not the feet of the angle bisectors. My guess is that this is a fairly common point of confusion for kids learning this topic:

(ii) the circumcircle

Here his solution was completely correct. He had a tiny bit of trouble in the beginning figuring out to construct the perpendicular bisectors, but he worked through that trouble fairly quickly.

It is always interesting to hear the ideas that kids have when they talk through a mathematical process. For me it was especially nice to hear that most of the ideas he’s learning as he works through this geometry book are sinking in pretty well.

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## Part 2 sharing Mathologer’s “triangle squares” video with kids

Yesterday we did a project inspired by Mathologer’s “triangle squares” video:

Here’s the project:

Using Mathologer’s triangular squares video with kids

Today we took a closer look at one of the proofs in the Mathologer’s video -> the infinite descent proof using pentagons that $\sqrt{5}$ is irrational:

Here are some thoughts from the boys on the figure and the proof. You can see from their comments that they understand some of the ideas, but not quite all of them.

Watching Mathologer’s video, I thought that the triangle proof about the irrationality of $\sqrt{3}$ and the proof of the irrationality of $\sqrt{2}$ using squares were something kids could grasp, but thought that the pentagon proof presented here was a bit more subtle. We may have to explore this one more carefully over the summer.

After discussing the proof a bit, I switched to something that I hoped was easier to understand. Here we talk about the different pairs of numbers that create fractions close to $\sqrt{5}$.

The boys were able to explain how to manipulate the pentagon diagram to produce the fraction 38/17 from the fraction 9/4 that we started with. From there the were able to also show that 161/72 was also a good approximation to $\sqrt{5}$:

Next we went to the computer to explore the numbers, and also to see how the same numbers appear in the continued fraction for $\sqrt{5}$.

In the last video we tried to do some of the continued fraction approximations in our head, but that wasn’t such a great idea. Here we finished the project by computing some of the fractions we found in the last video by hand.

I love Mathologer’s videos. It is amazing how many ways there are to use his videos with kids. Can’t wait to explore these “triangular squares” a bit more!

## Using Mathologer’s “Triangular Squares” video with kids

Last month Mathologer published an incredible video on what he calls “Triangular Squares”:

I’ve been meaning to use this video for a project for the boys ever since I saw it. Today I finally got around to watching it with the boys.

Here are their initial thoughts after watching the video:

Now we went through some of the ideas. First I asked the boys to try to sketch Mathologer’s argument that $\sqrt{3}$ is irrational. Then I asked what proof they would have given for that fact without seeing the video:

Next we explored the irrationality proof for $\sqrt{2}$:

Finally, we did a bit of exploration of the seeming paradox mentioned at the end of the video. That paradox is essentially -> the argument used to show that $\sqrt{3}$ is irrational seems to also show that 3 times a triangular number can never be a triangular number. BUT, there are lots of examples showing that 3x a triangular number is a triangular number. What’s going on?

So, another terrific video from Mathologer. His ability to shed light on advanced math topics for the general public is incredible. I love using his videos to help my kids see amazing math ides from new and beautiful angles!

## What a kid learning math can look like – incircles and circumcircles

This problem gave my son a lot of trouble this morning. It is from one of the challenge problem sections in Art of Problem Solving’s Introduction to Geometry book:

A triangle has side lengths of 10, 0, and 12, find the lengths of the radius of the inscribed and circumscribed circles.

This problem looks like a pretty plain vanilla math contest problem, but he’s not studying for math contests. He drew an amazing picture of the situation:

After talking about the problem a bit this morning we moved on to other things. When he got home from school we revisited it. Here are his thoughts on the incircle part of the problem:

Here are his thoughts on the circumcircle part of the problem:

Finally – and unfortunately interrupted by a phone call – here’s his explanation of how the project from today relates to a 3d printing project that we’d done previously

Definitely a fun problem to talk through, and really nice to see that he was able to explain the ideas in a problem that he’d really struggled with in the morning.

## Using the Infinite Galaxy puzzle from Nervous System to talk topology with kids

The boys and I spent yesterday working on the new Infinite Galaxy Puzzle from Nervous System:

Having finished it, I thought a project talking about some of the math behind the puzzle would be really fun for the boys.

Since the front cover of the puzzle says that it was inspired by the Möbius strip, I started today’s project talking about that shape:

Next we talked about the puzzle and what geometric / topological properties it has. The interesting mathematical question here is whether or not the puzzle is a Möbius strip?

It turns out the puzzle is projective plane!!

We spent the last part of the project today talking about the projective plane and a few other similar shapes.

Even without any of the math, this new puzzle from Nervous System is a really fun challenge. The mathematical ideas behind the puzzle move it from the “fun puzzle” real to the “blow your mind” realm, though!

I’m so happy to have found one of these puzzles at the Nervous System open house last weekend. What an amazing way to share some introductory ideas from topology with kids!

## 3d printing totally changed my approach to talking about trig with my son

For the last two weeks we’ve been playing with this book:

Our most recent project involved one of the pentagon dissections. My son wrote the code to make the shapes on his own. We use the RegionPlot3D[] function in Mathematica. To make the various pieces, he has to write down equations of the lines that define the boundary of the shape. Writing down those equations is a fantastic exercise in algebra, geometry, and trig for kids.

Here’s his description of the shapes and how he made the pentagons:

Next we moved on to talking about one of the complicated shapes where the method he used to define the pentagon doesn’t work so well. I wish I would have filmed his thought process when he was playing with the code for this shape. He was really surprised when things didn’t work the first time, but he did a great job thinking through what he needed to do to make the shape correctly.

Here is his description of the process followed by his attempt to make the original shape (which he’d not seen in two days . . . )

I’m so happy that he’s been interested in making these tiles. I’ve honestly never seen him so engaged in a math project. The original intention of this project was just for trig review, but now I think creating these shapes is a great way to use 3d printing to introduce basic ideas from trig to students.

## Playing with the nonagon tiles

Two of our recent project have involved studying a tiling of a nonagon from the book “Ernest Irving Freese’s Geometric Transformations”

Those two projects are linked here:

Using “Ernest Irving Freese’s Geometric Transformations” with kids

nonagon tiles

After school yesterday I had each of the boys make a pattern with the nonagon tiles and then build the two patterns that were in the book. The videos below show there work. My younger son went first:

Here’s what my older son had to say:

This project was super fun from start to finish. Hearing the thoughts from the boys after seeing the pattern initially was really fun. Building and printing the blocks was a nice geometry / trig lesson. Then having the boys play around with them made for a really satisfying end to the project. I hope to do more like this in the near future.

## Nonagon tiles

Last week we did a fun project using a pattern we say in “Ernest Irving Freese’s Geometric Transformations” by Greg N. Frederickson:

Using “Ernest Irving Freese’s Geometric Transformations” with kids

I thought it would be fun to make some of the tiles – especially since my older son is studying trig right now. The tiles finished printing overnight:

Last night my son and I talked about how you could make these tiles, with a focus on the trig and algebra required to define the shapes.

Here’s the introduction to the topic:

Now we talked about how to define the kite shape in the tiling. This involves talking about 40 and 50 degree angles:

Finally, we talked through the last part – finding the final point is pretty challenging. Turns out, though, that we don’t have to find the coordinates of the point because we can write down the equation of the top line pretty easily:

I’ve been happily surprised that 3d printing is a fun way to help kids explore 2d geometry. I’m excited to have my son try to make some other tiles from the book on his own for our next project.

## Using “Ernest Irving Freese’s Geometric Transformations” with kids

A few weeks ago we got this book in the mail:

We used the book for one fun project already:

Playing with geometric transformations

Today we were really short on time in the morning, but I still wanted to do a project before we ran out the door. I’d been hoping print some of the shapes from the book, so today for a quick project we looked at one of the transformations.

Here’s the introduction to the shape and some of the thoughts that the boys had:

Next we tried to understand some of the details about the shapes – could we understand anything about the lengths of the sides or the angles?

I think that we now have enough information to make the tiles. It was nice that a little bit of trig came up since that’s what my older son is studying right now. Not sure if I’ll have time this weekend or not, but we’ll hopefully be able to do a project with the 3d printed tiles in the next week.

## Talking about medians

My younger son has been studying Art of Problem Solving’s Introduction to Geometry book this year. He’s been doing most of the work on his own, but I check in every now and then.

Today he was working in the section about medians, so I thought I’d ask him what he’s learned.

Here were his initial thoughts.

In the last video he was struggling to remember how to prove that the 6 small triangles formed when you draw in the medians of a triangle all have equal area. Here I gave him a hint and he was able to finish off the proof:

We wrapped up with a short discussion about the lengths of the medians.

Overall a fun discussion. I’m a big fan of the Art of Problem Solving books and am happy that my son is enjoying working through their geometry book.