Horizontal Cross Section of a Triangular Pyramid

This example demonstrates how a horizontal plane cuts through a triangular pyramid (tetrahedron), showing the changing shape of the cross-section as the cutting height varies. The core mathematical concept is solid geometry, specifically cross-sections and parallel plane properties: when the horizontal section is parallel to the base, the cross-section is a similar triangle; when tilted or at different heights, it becomes a trapezoid or other quadrilateral. Construction details: using GeoGebra commands Point to define vertices A, B, C, D, Pyramid to build the tetrahedron, Plane to create the horizontal cutting plane p, and IntersectPath to compute the intersection curves between the plane and each face, generating section polygons poly1 and poly2. By moving the horizontal plane (involving objects E, F, G, H, I, J, etc.), users observe the dynamic transition of the cross-section from a triangle to a trapezoid. Suitable for high school or early college students learning cross-section properties in solid geometry, and for teachers demonstrating spatial visualization in class.