Many geometric properties can be effectively and dynamically illustrated through paper-folding activities. Here is one. We think students will enjoy folding their own examples.

Review with students that the measure of a “straight” angle is 180°. Demonstrate this using a protractor.

OBJECTIVE

To show that the sum of the measures of the angles of a triangle is 180°.

MATERIAL

A triangular piece of paper.

DIRECTIONS

	1. Begin with a triangular region ABC.
	2. Fold vertex C onto side BC so that the crease passes through vertex A.
	3. The crease AD is the triangle’s altitude through vertex A.
	4. Fold vertex A onto point D.
	5. Fold vertex C onto point D.
	6. Fold vertex B onto point D.
	7. Show that the sum of the measures of the angles A, B, and C is 180°.

ANALYSIS

The three angles have been folded onto a straight-line segment. Hence the sum of their measures must be 180°.