Supplementary Angles: Visualizing a Pair That Always Sums to 180°

This applet demonstrates the geometric meaning of supplementary angles: as one side of an angle rotates, the paired angle remains supplementary so that the two angles always add up to 180°. It centers on angle measurement, supplementary-angle relationships, and how angle measures persist under geometric transformations. The construction uses GeoGebra commands reflected in the build summary: Line to set up the relevant lines, Point to place key vertices and auxiliary points (A, B, C, D, T1/T2/T3), Ray to define the two sides of the angles, and Angle to measure and display the angles (α, β, γ and corresponding labeled angles). Rotate is used to animate the turning of a ray, letting learners observe continuously that the supplementary condition holds. Suitable for middle/high-school geometry lessons and for interactive self-study focused on angles and their relationships.