On the screen, we see a triangle ABC in blue color.[br]Observe how the shape and angles of this triangle ABC change as we move the points A, B and C. We also see line ECD which is parallel to line AB

Questions to think about[br]1. What is the sum of the angles <ECA+<ACB+<BCD? [br]2. What is the value of angle BCD? How does it compare it with angle ABC?[br]3. What is the relation between angles ECA and angle CAB?[br]4. Now, figure out how we can determine that the sum of the angles of the triangle is always 180 degrees.