Triangle Types

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Students will identify four types of triangles; right, isoceles, scalene, and equilateral.

Big Idea

Not all triangles are created equil...lateral!


15 minutes

This lesson is designed to get students to recognize four types of triangles; isosceles, scalene, right and equilateral.  Although identifying triangle types is not directly listed in the fifth grade geometry standards, students need a familiarity with angle types in order to set the background for classifying shapes based on their attributes and building a hierarchy of their properties.

I open the lesson by playing the students a short youtube video showing the triangle song.  The video highlights all four types of triangles.  Short youtube clips are a great way to get students engaged in the upcoming lesson.  I follow the video up by asking students to share their observations of the video.

Can you share with your neighbor what the four types of triangles are?  Try giving them a definition for one of those types of triangles. 

After recapping the video in their groups I have the students create a foldable with the four definitions of a triangle.  I have included a video of how to set up this foldable.  It is a pretty easy one.  Foldables are a great way to record new vocabulary for the students.  A foldable is a more engaging way to write information as compared to a standard list format. 



20 minutes

 In order to practice their new knowledge of types of triangles, I have students play a game that asks them to identify types of triangles based on the length of their sides.  The students use dice to create three lengths of sides and then based on those lengths they have to determine which type of triangle they created.  Although this activity omits examining the angles of the triangles, I think it does a great job of focusing on the length of the sides being similar or different.

I explain the directions of the game to the students and give each one a gameboard.

Who can explain the directions to me before we get started?  Please find a partner and get three dice from the bin.  Find a spot in the room to work with your partner.  

As the students begin playing the game I monitor partnerships for following game instructions and and understanding of the three types of triangles that are highlighted in it.  I give the students about 15 minutes to play and then ask them to return to their seats. 

Let’s talk about this game.  Were there any strategies you used in this game?  What was the hardest type of triangle to get?  Can someone give me an example of an equilateral triangle they rolled?  How about a scalene?  Isoceles?

During the group discussion I ask students to relate information of the game back to the definitions we created early in the lesson.


15 minutes

To wrap up this lesson on triangles, I ask that students think a little more abstractly about the types of triangles they learned.  I will display some examples of triangles and ask students to identify the types of triangles in the illustration.  By being able to identify the triangles visually instead of in number form, I hope to create a deeper conceptual knowledge of triangle types.

Alright, keep your math brains turned on and put it in triangle mode.  I would like you to first think for a minute quietly about what you see.  Share with your neighbor some of your observations.

I then ask for several students to come up to the board and share their observations.  I want students to not only be able to identify the triangle but also provide evidence of their conclusion.

What type of triangle did you observe in this illustration?  How do you know it is that type of triangle? 

Although we don’t have rulers up at the board, my expectation is that they will estimate and defend the size of the sides.