Sum of the Angles of a Triangle  

READ the instructions below and INTERACT with the GeoGebra applet.

  1. Move points A, B and C to change the triangle’s shape and size.

  2. Next, move the green point on the slider completely to the right. What is the sum of the three angles?

  3. Repeat steps (1) and (2). What do you observe about the sum of the angles?

  4. What mathematical reason can be used to justify the two green-shaded angles are congruent?;
    the two blue-shaded angles are congruent?;
    and the two brown-shaded angles are congruent?
 
Sorry, the GeoGebra Applet could not be started. Please install Java 1.4 (or later) to use this page.