Terrific Triangles

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SWBAT understand how triangles can be classified.

Big Idea

You will use properties to classify triangles.


5 minutes

To review angles from yesterday, I ask students to hold their arms to make angles. I call out straight angle, and students hold their arms out straight. I call out right, and students hold their arms as such. We continue to name all angles, including obtuse and acute angles as well. To build on previous knowledge, we are now moving onto classifying triangles. We discuss the prefix tri-, and create a list of other words that have tri- as a prefix. This incorporates grammar into instruction. When students see this prefix again in a reading passage, and encounter an unfamiliar word, they can use a root word/prefix/suffix to help to decode the word.

Guided Practice

20 minutes

I pose a problem to students: Robbie draws a triangle with two sides that are the same length. The triangle has one obtuse angle. How can Robbie classify the triangle? I let students hypothesize how triangles can be described (classified). Students determine that triangles can be classified by the length of its sides. I explain to the students that triangles have a "first name" and a "last name", just like them. Their first name is either: acute, right, or obtuse. Their last name is either: scalene, isosceles, or equilateral. We work through each triangle and highlight the name. We mark each side of the scalene triangle with 1, 2, and 3 lines to indicate that each side is a different length. We mark each side of the isosceles triangle with 1, 1, and 2 lines to indicate that 2 sides are the same. I also have students draw an eye to represent the "i" in isosceles. We mark each side of the equilateral triangle with 2, 2, and 2 lines to indicate that all of the sides are the same length. We continue this pattern for the acute, right, and obtuse triangle as well. In table partners, students analyze details, and determine how Robbie can classify the triangle. Students determine that Robbie's triangle is an obtuse triangle because it has one obtuse angle.

We view Angle This Way to introduce classifying triangles. The students view snack foods, which they can relate to, and then view non-examples and examples of the various types of triangles. I allow students to use sign language to show me which choice they think is correct. After a countdown, 5-4-3-2-1,... students hold up their letter signal. This allows me to formatively assess who understands and who does not. Students can then use the SmartBoard to sort the triangles into the appropriate category.

Independent Practice

25 minutes

Students review what we have discussed by working with their partner to color code, draw lines on sides, and discuss the process together.

Students, in partners, sort (12) triangles into categories: Right Triangle, Scalene Triangle, Acute Triangle, Isosceles Triangle, Obtuse Triangle, Equilateral Triangle.

Sudents classify (6) triangles with both a "first" and "last" name. Students also solve a word problem about the Louvre Museum in Paris, France; they classify the triangle on the front of the Louvre Pyramid by the lengths of its sides and the measure of its angles. The students determine that it's a scalene acute triangle. Students report out their answers, and I review.


10 minutes

In The Greedy Triangle, a little triangle who's tired of being a triangle gets a side added and an angle added, which transforms him into a quadrilateral. He's not satisfied though, and then adds another side and another angle, and another, and another, until he's almost a circle. Here, students are exposed to many different types of triangles; this will review what they just worked on with their partner practice activity.

This book contains vivid pictures, and the pages are full of graphics. In reading this book, students are exposed to mathematical language in context which they can then use in their own language.