Monthly Archives: January 2019

Similar Triangles and a Self-Checking Physical Challenge: Mirror, Mirror on the Floor

Yesterday in Geo, I took some advice from Dan Meyer “You Don’t Have To Be The Answer Key” and set up a fun self-checking activity based on this blog post, Eye to Eye. The premise: place a sticky note on the wall, and then place a tiny mirror on the floor between you and the wall so that you can glance into the mirror and see the sticky note. The catch is that you can’t just stand and move around until you can see it, you need to place yourself, open your eyes and look, and see if you see it! The mirrors I found in the physics department were probably 3 inches in diameter, which was perfect for a little bit of precision, but enough wiggle room that this worked. It was fun because students would be really excited that it worked! And if it didn’t, they would just go back and check their calculations without needing direction from me.

Here were the three situations (I gave them the text and then they needed to show me their diagram kind of like the ones I drew below, and calculations before they were allowed to try it physically):

1. The sticky note is 7 feet up on the wall, and the mirror is 3 feet from the wall. Where should you place yourself so you can see the sticky note in mirror?


2. Now place yourself 5 feet from the wall, and place the mirror 4 feet from you. Where should you place the sticky note so that you can see it in the mirror?


3. Now place yourself 5 feet from the wall, and place the sticky note 3 feet up. Where should you place the mirror so you can see the sticky note in the mirror?


These got more difficult as they went a long, and kids did a great job with the last one solving it in a ton of different ways (most using some sort of x and 60-x on the bottom). My favorite was a boy who measured his eyes to be 63 inches off the ground.

“Well, I’m 63 inches tall, so the ratio of my height to the sticky note is 63:36, which simplifies to 21:12, but 21+12 = 33, so if I break the 60 inches on the floor into 33 pieces and then multiply that by 12, that’s how far I should place the mirror from the wall.”


Lesson Outline: Origami Construction of Octagon

I did this kinda fun hands on intro to the angles in a regular polygons earlier this year and I wanted to share. It was inspired/thieveried by an Illustrative Mathematics lesson that I can’t find on the internet now (I think their curriculum is about to come out, which I’m excited about, but maybe some of their stuff online going away) and this blog post from Jennifer Wilson, so nothing new, but I figured I’d amplify and give my own thoughts.

  1. Fold the Octagon:
    • Take a square piece of paper, and halve it by folding one edge to the other edge across the way, unfold, and do it the other way too. Then halve it along the two diagonals too.
    • Then fold all the halfway lines between those lines. Easiest way to do this is fold along the current lines until you get one of those eighth triangles, and then fold the folded edges to each other (see picture).
    • Unfold. Then find the points indicated in the diagram and make a fold on the line between those two points (my diagram is a bad construction of this, they shouldn’t be exactly quartering the top).
  2. Any conjectures about the shape? We talked through some of these and talked through why they might be true. Good reasoning involved the fact that when you fold something onto something else, it makes it congruent.
    • It’s an octagon.
    • It’s a REGULAR octagon.
    • There are four kites in the figure (can you see them?)
    • There are 8 isosceles triangles.
    • The last fold we did made a 90 degree angle with the other fold.
  3. Now physically label the measure of all of the angles. Like, with a pencil and not your brain. Justify your thoughts.
    • This was interesting because there seemed to be two ways to go:
      • Some started with the corners. Since that angle is folded in half, those are two 45 degree angles. And the last fold makes two 90 degree angles because the angles are folded on top of each other (congruent) but also along a line (supplementary) so have to be 90. Then the other angle in those corner triangles is 45 degrees…. and go from there.
      • Some started in the middle. Since we folded all 16 of those central angles on top of each other, they have to be congruent, and since they add up to 360, they need to be 22.5 degrees. Then lots of places to go from there either with isosceles triangles or right triangles.
    • After kids spent 10 minutes or so labeling on their own or with a partner, we took turns sharing some ideas. Some kids talked through their thinking and we all gave critiques. It was a good switch between individual-full class modality too.
  4. Now, draw a STAR where there is an exterior angle and a HEART where there is an interior angle.
    • We had just learned what these and had learned how to calculate them so I wanted to see if they could find one on this complicated diagram. This was harder than I thought it would be, which meant it was a good use of time!
    • Do the measures of these angles match with the equations we figured out to calculate them? (Yes! 45=360/8 and 135=180-45=180(8-2)/8).

This took maybe 40 minutes (I can’t tell from my lesson plan if that’s right), but was great! It was physical and exploratory and was full of fun geometrical arguments that are based in transformations, but also had a nice, concrete, angle-labeling component for kids who prefer numerical lessons. This gets my 😀 of approval!!!

Coding in Geo: Snap! Regular Polygon Art

One of our department’s curriculum redesign goals is to incorporate a bit of coding into our curriculum, and the place they decided to place that was Geometry. We have been coding in Snap!, a block based coding platform really similar to Scratch. Block based means that students aren’t typing commands, but rather dragging and dropping them into lists to make programs. The advantage: no syntax errors, or spelling errors that are the bane of every beginning coder (wHy WoN’t It RuN?!? Well, because you have “Power” written here and “power” written here and the computer doesn’t know that you think those are the same thing). The disadvantage: it’s a bit clunky, in particular the saving and sharing system.

After an initial day where the kids explored by trying to get the program to write out an English letter, we then had them work to code in a regular polygon, something that would teach them both about loops and variables, and practice calculations of interior and exterior angles etc. Here is the packet of instructions we used, with much inspiration/petty theft from Dan Anderson (@dandersod, his conference materials).

Then, the instructions I gave them were to make a beautiful piece of art that shows of their understanding of regular polygons, coding loops and variables. Your code had to run in one click. The results were SUPER cool, and the kids loved it! Here are some below:

scratch projects 18

Sorry they are so small, but there are so many cool ones, this isn’t even all of them! Can’t wait to hang these up in the classroom.

Along the way, without me showing are really them needing to, kids figured out how to: incorporate sounds, incorporate input from the user, use randomness, and one kid figured out his own version of the sine function. I also had them write a written description of how their code works and what their artistic inspiration was, and they were adorable. I could tell how proud some kids were of their work! <3.