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