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.
- Explain (in writing) how to use circles to locate the point where the sides of a triangle with known side lengths should meet.
- Use manipulatives to justify when it is not possible to make a triangle with three given side lengths.
- Use manipulatives to show that there is a minimum and maximum length the third side of a triangle could be, given the other two side lengths.
Let’s build more triangles.
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.
- I can reason about a figure with an unknown angle.
- I can show whether or not 3 side lengths will make a triangle.
Print Formatted Materials
For access, consult one of our IM Certified Partners.