Unit 2 – Tools of Geometry

13 Oct

Last year we started with this unit. This year, we decided to start with our unit on Rigid Motions, since students have some familiarity with transformations from middle school. I noticed that students were much more willing to look for regularity in repeated reasoning and make sense of problems and persevere in solving them during the Tools of Geometry unit because they had some practice with those practices during the last unit.

CCSS-M Standards:

G-CO 1

Know precise definitions of angle, circle, perpendicular line, parallel line, and line segment, based on the undefined notions of point, line, distance along a line, and distance around a circular arc.

Prove theorems about triangles. Theorems include: measures of interior angles of a triangle sum to 180°; base angles of isosceles triangles are congruent; the segment joining midpoints of two sides of a triangle is parallel to the third side and half the length; the medians of a triangle meet at a point.

G-CO 12

Make formal geometric constructions with a variety of tools and methods (compass and straightedge, string, reflective devices, paper folding, dynamic geometric software, etc.). Copying a segment; copying an angle; bisecting a segment; bisecting an angle; constructing perpendicular lines, including the perpendicular bisector of a line segment and constructing a line parallel to a given line through a point not on the line.

G-CO 13

Construct an equilateral triangle, a square, and a regular hexagon inscribed in a circle.

G-C 3

Construct the inscribed and circumscribed circles of a triangle, and prove properties of angles for a quadrilateral inscribed in a circle.

I can statements:

Level 1: I can use basic terms in geometry: point, line, plane, polygon, inscribed, circumscribed.

Level 2: I can create basic geometric objects based on their definitions: median, angle bisector, altitude, perpendicular bisector.

Level 3: I can use tools and methods to perform formal constructions.

Level 4: I can solve problems using points of concurrency.

We learned about the tools of geometry through a few applications – Placing a Fire Hydrant & Locating a Warehouse, both of which come from Illustrative Mathematics. We used the Special Segments activity from Geometry Nspired, and we used The Shipwrecked Surfer problem published in the August 2006 Mathematics Teacher. I plan to write a separate post about each of the main lessons.

We spent the rest of our time on constructions – really paying attention to what is congruent as we did each construction. I wanted the constructions to come from a context, instead of just passing out a list of steps for a construction and asking students to do the construction because it was on our list of standards.

I have already looked at the student reflections for this unit. While the majority of students still believe that the practice they most used was make sense of problems and persevere in solving them, the second most used practice was use appropriate tools strategically (which, for a unit on constructions, means that students really thought about which practices they were using and didn’t just mark the first one on the list).

I asked students whether any of the content seemed like repeats of previously learned material.

  • Everything that I learned this unit was completely new to me.
  • I felt that I didn’t already know any of the material covered; it all seemed new to me.
  • I didn’t know any of this but I learned a lot!
  • Absolutely none of them were repeats for me. There were parts of each unit I’d already learned and heard about, but the overall idea of each lesson was new to me.
  • All of the lessons were excellent introductions into learning about and exploring circumcenters, centroids, orthocenters, and incenters. They required the usage of different techniques and critical thinking to achieve the answers. Finding these various properties of triangles required usage of terms and theorems from the past chapter about distance, perpendicular bisectors, justifications by measuring, etc.
  • Most of the information this unit was new. There were terms that I was already familiar with such as perpendicular and bisector, but I didn’t know all of the information I could gain just by saying that a line is a perpendicular bisector, an angle bisector, an altitude, or a median.

Which lesson helped you the most in this unit? Most students said that The Shipwrecked Surfer helped them the most.

  • The shipwrecked surfer really helped understand all of the different segments and points and centers. For us to have to bring together the median, altitude, angle bisector, and perpendicular bisector and their corresponding centers, so to say, showed me how they all work and when they can all be the same. It made more sense to see it like that.
  • I would say that 2A, 2B, and 2D helped the most because they were realistic situations.

What did you learn during this unit?

  • I learned how to use the incenter, circumcenter, and centroid to solve real-world problems.
  • I learned a lot that I didn’t know. I really enjoyed this unit because of this. I can easily find perpendicular bisectors, altitude, medians, and all the points of concurrency and other things we learned in this unit.
  • Other than the various terms and the application of said terms, I learned not to immediately give up when a problem doesn’t practically solve itself.
  • During this unit I learned many new mathematical terms. I also learned how to classify a triangle by using information other than the degree of it’s [sic] angles.
  • I have learned basically everything that the teacher has taught me. I didn’t know much of it, but I knew enough where I didn’t feel completely lost. I learned a lot of vocabulary words.

I think it is significant that the last student said he “learned a lot of vocabulary words”. When I talk about teaching geometry through exploration, I often have teachers ask about the vocabulary. For many, geometry is all about vocabulary – and assessing whether students know definitions and notation. The biggest difference in how I was taught geometry and how I teach geometry is that we don’t start with the vocabulary. We work through meaningful tasks (the students’ words – not just mine) in which the vocabulary will surface. When the vocabulary does surface, I make a big deal about it. I write the word down on the board. I suggest that they write the word down in their notes. I ask what the object will buy us mathematically, and I sometimes find myself saying how you would see the word defined or the object notated if you were to look in a textbook. I have a list of vocabulary in my lesson plans that needs to surface in the lesson, but that list shows up for the students one word at a time, as we get to it in our exploration, instead of as a list of words to define from the glossary before (or even worse, during) class.

And so the journey continues, right or wrong …



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5 responses to “Unit 2 – Tools of Geometry

  1. Travis

    October 14, 2013 at 4:04 am

    I think I will try the reversal you are espousing….lots of questions leading up to a p.b. of a segment construction, and then ‘naming’ what appears to be true. Then some proving using the newly formed vocabulary along with previous terms.

    • jwilson828

      October 14, 2013 at 10:41 am

      Keep us posted on how it goes, Travis.

  2. Karen

    October 16, 2013 at 6:40 pm

    I would like to know how to get the shipwrecked surfer. I cannot find any archived Mathematics Teachers.

    • jwilson828

      October 16, 2013 at 9:37 pm

      Hi, Karen.
      I was working on a post about that lesson last night. Your comment spurred me to get it finished. There is a link to the original paper that has been posted by the author of the article in my new blog post. I’d love to find out how it goes if you use it.

      • Karen

        October 16, 2013 at 10:17 pm

        Thanks! I’ll let you know what happens.


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