About angles in a triangle

What can we learn from investigation of the angles in a triangle? This set of activities focuses on the angle relation within a triangle. You may find different ways to explore the angle sum property. By comparing these different ways of exploring and/or proving this property about the sum of interior angles, we may also appreciate different connections with other geometric properties and methods of reasoning. This collection is intended to suggest a variety of tasks beyond the common choice between a simple experiment for discovery/verification and a formal deductive proof. The learning objectives may include building concepts about angles in a geometric figure, noticing properties and relations, connecting intuitive understanding and reasoning.