Reflection in the y-axis

① This GeoGebra example dynamically demonstrates the reflection (mirror) transformation of a geometric figure (such as a quadrilateral) about the y-axis, visually showing the symmetric relationship between the original shape and its image. ② The core mathematical concept is axial symmetry in the plane, involving properties of reflection: the segment connecting corresponding points is perpendicularly bisected by the axis, and the shape and size remain unchanged while orientation reverses. ③ Construction highlights: from the summary, commands such as Intersect, Vector, Segment, and Angle are used, creating objects b, a, B, C, u, C', u', text1, text2, D, c, d, α, e, β, where C' and u' likely represent reflected points and vectors, with angles and segments used to verify reflection properties. ④ Suitable for middle and high school students learning plane geometric transformations, and for teachers demonstrating symmetry concepts in class, helping to connect algebraic and geometric aspects of coordinate-axis reflection.