Geometric Demonstration of Trigonometric Addition Formulas

This GeoGebra example dynamically illustrates the geometric meaning of trigonometric addition formulas (e.g., sin(α+β) and cos(α+β)). Core mathematical concepts include angle addition, trigonometric functions on the unit circle, vector rotation, and triangle side-angle relationships. Construction highlights: using GeoGebra commands such as Intersect, Circle, Rotate, Angle, and Polygon, the applet creates objects A, c_1, α, I, B, β, γ, C, δ, poly1, and multiple text labels (texte1 to texte9). By rotating points and finding intersections, it constructs triangles related to the sum of angles and displays the formula derivation via text. Suitable for high school or pre-university students learning trigonometric identities, vector rotation, and geometric proofs, as well as for teachers demonstrating how algebraic formulas arise from geometry.