Lesson 7

In this lesson, students experiment with constructing triangles given 2 or 3 side lengths. They start by working with cardboard strips and metal fasteners, as in the previous lesson. They discover that there are some combinations of lengths that do not make a triangle. Then students move toward using a ruler and compass, seeing that it recreates the functionality of the cardboard strips and metal fasteners more efficiently. The purpose of this transition is to help students move toward a mental understanding that does not depend on physical objects, helping them work toward the understanding that in a triangle the sum of any two sides must be greater than the other side.

When students use repeated reasoning with specific cases to formulate a general rule about which side lengths are possible for triangles, they engage in MP8.

Print the Swinging the Sides Around blackline master. Prepare 1 copy for every 2 students.

Students will also need the cardboard strips and metal paper fasteners from the previous lesson, as well as access to geometry toolkits and compasses.

Note: If using the digital version of every activity, these supplies will not be needed.