Open Middle Triangle Angle Sum Explorer

This example demonstrates an “open middle” activity for discovering and verifying the interior angle sum of a triangle through guided construction. The key ideas are triangle interior angles, angle construction and measurement, additive angle reasoning, and how equal angles arise from geometric transformations. It is built by using GeoGebra commands such as Intersect (to find key intersection points), Point (to create construction points), Rotate (to generate rotated configurations), Ray and Angle (to draw and measure rays/angles), Polygon and Segment (to assemble triangle/polygon geometry), and Length/Distance (to compute geometric quantities that support constraints). Conditional logic with If and on-screen annotations via text objects (for α, β, γ and labels) make the reasoning interactive and verifiable. Suitable for middle and high school geometry lessons, inquiry-based learning, and classroom explorations focused on angle-chasing and proof-by-construction.