Investigating Geometry with GeoGebra

Henrico County Public Schools

What is the sum of the interior angles of a triangle?

READ the instructions on the right and INTERACT with the GeoGebra applet below.

Sorry, the GeoGebra Applet could not be started. Please install Java 1.4 (or later) to use this page.

Investigation Steps

Click on the reset icon icon to reset the diagram.

 Step 1 Close 
  1. Move points A, B and C to change the triangle’s shape and size.
 Step 2 Close 
  1. Next, move the green point on the slider completely to the right.

  2. What is the sum of the three angles?

 Step 3 Close 
  1. Repeat steps (1) and (2).

  2. What do you observe about the sum of the angles?

  3. How can you prove this is true for any triangle?
 Questions Close 
    Submit your written response to the following questions to your instructor.

  1. What mathematical reason can be used to justify the two green-shaded angles are congruent?;

  2. the two blue-shaded angles are congruent?;

  3. and the two brown-shaded angles are congruent?