Education Technology

Angle Sum in Triangles Proof using Rotation and a Parallel Line

Published on 06/09/2008

Activity Overview

This investigation uses Cabri Jr. and a cleaver rotation of a triangle to "prove" that the angles in a triangle add up to 180. This could be used to reinforce triangles and paralled lines as well as introduce the concept of rotating an object.

Before the Activity

Print out and review the word document Angle Sum Measurement. This is an easy construction and the investgation may only take 20 minutes or 45 minutes depending on the students familarity with Cabri Jr

During the Activity

The student will know that the angles in a triangle add up to 180. The point of this investigation is get them to "prove" it by realizing that after the rotation the three angles by point A are the three diffenent angles of the triangle. Since the three angles add up to a straight line, the sum must always be 180.

After the Activity

The Cabri Jr file (app var) attached demonstrate another way to do the rotation using angles in circles. The rotation is controled by moving the central angle in the circle. This file has a separate circle for each rotation and can make a nice demonstration.