Education Technology

Exterior and Interior Angle Theorem

Published on 06/09/2008

Activity Overview

In this activity, students investigate the relationship between the exterior angle of a triangle and its remote interior angles. They study the Triangle Sum Theorem and the Exterior Angle Theorem.

Before the Activity

Install the Cabri™: Jr. App on the students' graphing calculators using one of these two methods:

  • TI-Connect™,  a TI Connectivity Cable, and the Unit-to-Unit Link Cable
  • TI-Navigator™  "send to class" feature
  • See the attached PDF file for detailed instructions for this activity
  • Print pages 11 - 13 from the attached PDF file for your class
  • During the Activity

    Distribute the pages to the class.

    Follow the Activity procedures:

  • Construct a triangle and label its vertices
  • Measure the angles of the triangle and record their sum
  • Drag one of the vertices around the screen and verify whether this sum remains the same
  • Observe that the sum of the measures of the interior angles of the triangle is 180 degrees
  • Extend a side of the triangle into a line and label a point on this line
  • Measure the exterior angle
  • Measure the remote interior angles and calculate their sum
  • Tabulate and compare the measure of the exterior angle and the sum of the remote interior angles
  • Observe that the measure of the exterior angle is equal to the sum of the measures of the remote interior angles
  • Drag one of the vertices around the screen to alter the triangle
  • Record the values of the exterior angle and the sum of the remote interior angles in the table
  • Drag to alter the triangle and measure the angles
  • Observe that the relationship between the exterior angle and the sum of the remote interior angles remains the same
  • After the Activity

    Review student results:

  • As a class, discuss questions that appeared to be more challenging
  • Re-teach concepts as necessary