The angle measures of a triangle sum to 180 degrees.
Move the points A, B, and C to create any triangle you'd like. (Note: for the applet to work do not move point B on the opposite side of line segment AC)
Once you are satisfied with your triangle, click Show Line AB to begin the proof and continue through the checkboxes.
With each checkbox, ask yourself if you agree with what the box is showing. Can you guarantee this will always be true?
What Theorem proves that the red angles are congruent, and the green angles are congruent?