# Activities

• • • ##### Subject Area

• Math: Geometry: Triangles

• ##### Author 9-12

60 Minutes

• ##### Device
• TI-83 Plus Family
• TI-84 Plus
• TI-84 Plus Silver Edition
• ##### Software

Cabri Geometry™
TI Connect™

• ##### Accessories

TI Connectivity Cable

• ##### Other Materials
This is Activity 3 from the EXPLORATIONS Book:
Exploring The Basics Of Geometry With Cabri

## Exterior and Interior Angle Theorem

#### 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.

• 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