Challenge 2a: Reflection in the line y = a

① This example demonstrates the mirror reflection of a polygon (rectangle) across a horizontal line y = a, showing both the original and reflected figures with corresponding vertices. ② The core mathematical concept is axial symmetry (reflection) in plane geometry, involving the reflection line, corresponding points, congruent figures, and coordinate relationships. ③ Construction highlights: using GeoGebra commands Point to create points A, B, C, D, E; Polygon to form rectangle poly1; OrthogonalLine to draw perpendicular auxiliary lines; and Mirror to reflect points, edges, and the whole polygon, producing images C', D', E', poly1', while keeping the reflection axis e and auxiliary lines c, d, f. ④ Suitable for middle or high school geometry classes, this interactive challenge helps students intuitively understand properties of reflection and explore how the reflected figure's position and coordinates change.