Geometry Tutor: Classifying and Defining Triangles and Their Parts

Here are some ideas of how to introduce the triangles unit from Geometry to 9th graders.
Here are some lesson ideas dealing with classifying triangles and their parts.

Geometry Triangles 55 minutes

Congruent Triangles Lesson #1: Classifying and Defining Triangles and their parts

Context

This is the first lesson of the unit. A lot of information in this first section is strictly definitions, so most students will be fairly unfamiliar with some of the terms discussed in this lesson. However, students will have to set up some pretty simple algebraic equations to find the measures of missing angles in triangles. Students used this algebra in Algebra I and in the first three chapters of Geometry.

Objectives

Students should be able to classify triangles by sides and angles.

Students should understand the parts of triangle that make up its sides and angles.Resources, Media and Technology

- Class set of rulers and protractors

Procedures

Day 1

1. Class will begin with an explanation of the pretest. Then I will give out the pretest.

2. After all tests have been handed in, I will start the unit by drawing types of triangles on the board and I will ask the students to see if they can come up with any of the names off the top of their head.

3. For the ones they don't know, I will them and write all the types of the triangles on the board. I will tell them this is one way to classify triangles and that is by sides.

4. Next, I will do the same thing for classifying triangles by angles. I will emphasize there are TWO ways to classify triangles: by sides and angles.

5. Next, I will draw an isosceles and right triangle on the board. I will use these diagrams to define the legs, hypotenuse and bases of triangles.

6. After defining these, I will draw some triangles on the board and I will give the students a moment to think of what type of triangles they are. Then, I will call on students to give me the names of the triangles.

7. Next, I will ask the students if they know how many degrees the interior angles of a triangle add up to. I will write 180 degrees on the board and I will make sure to emphasize that fact. Next, I will draw a right triangle on the board and I will call on someone to come up with the rest of the interior degrees of the triangle.

8. The last topic I will cover is the exterior angle theorem. First, I will show a triangle with two of the interior angle measures filled in, and I will ask the students to come up with the nonadjacent exterior angle by any means necessary. They will hopefully find the third interior angle first and then use supplementary angles to find the exterior angle. I will then point out what the exterior angle theorem says. I will then work through a more complicated exterior angle theorem problem where they will have to setup and algebraic expression.

9. Finally, the last thing I will do is show the students how to use a protractor, which they will have to do in their homework. Hopefully, some of the students already know how to use a protractor and they can help one another if needed.

10. The homework will be given as a class assignment the following day.

Day 2

1. I will once again remind them of how a protractor works for those who have forgotten. I will allow for them to work in pairs if they like. I will hand out the rulers and protractors to every student. I will be going around the room and checking to see if everyone is working and to see if they are doing the work correctly.

2. As I am doing this, Mr. Robertson will be calling on students one at a time to go over last chapter's test with them.

3. I will work with some of the student's individually if they are still having trouble using the protractor. The worksheet should take the students the majority of class.

4. The students are required to turn in the worksheet by the end of class for me to grade.