Lesson 7

Building Polygons (Part 2)

Lesson Narrative

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.

Learning Goals

Teacher Facing

  • 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.

Student Facing

Let’s build more triangles.

Required Preparation

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. 

Learning Targets

Student Facing

  • I can reason about a figure with an unknown angle.
  • I can show whether or not 3 side lengths will make a triangle.

CCSS Standards


Print Formatted Materials

Teachers with a valid work email address can click here to register or sign in for free access to Cool Down, Teacher Guide, and PowerPoint materials.

Student Task Statements pdf docx
Cumulative Practice Problem Set pdf docx
Cool Down Log In
Teacher Guide Log In
Teacher Presentation Materials pdf docx
Blackline Masters zip

Additional Resources

Google Slides Log In
PowerPoint Slides Log In