Sam's Tetrahedron: Minimum Volume Tetrahedron from Tangent Plane on an Ellipsoid

Sam's Tetrahedron: Minimum Volume Tetrahedron from Tangent Plane on an Ellipsoid
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2017-04-02 06:07
This GeoGebra example demonstrates the problem posed by Sam Shah: given an ellipsoid x²/A² + y²/B² + z²/C² = 1, choose a point D on the surface, construct the tangent plane at D, and form the tetrahedron bounded by this tangent plane and the three coordinate planes. The goal is to explore the minimum volume of such tetrahedra as D varies. Core mathematical concepts include tangent planes to implicit surfaces, partial derivatives, volume of tetrahedra in 3D space, and optimization. The construction uses GeoGebra commands: PointIn to place D on the ellipsoid, Derivative to compute partial derivatives, OrthogonalPlane to create the tangent plane, IntersectPath and Intersect to find intersections with coordinate axes, Pyramid to build the tetrahedron, and Textfield/InputBox for interactive adjustment of parameters A, B, C. This is suitable for university-level calculus, multivariable differentiation, or spatial geometry courses, and serves as a vivid visualization for optimization problems and tangent plane applications.

GeoGebra Commands & Parameters

  1. PointIn(a) → D
  2. Derivative(f, y) → fy
  3. Derivative(f, x) → fx
  4. OrthogonalPlane(D, u) → b
  5. IntersectPath(b, z = 0) → g
  6. IntersectPath(b, x = 0) → h
  7. IntersectPath(b, y = 0) → i
  8. Intersect(g, h) → E
  9. Intersect(g, i) → F
  10. Intersect(h, i) → G
  11. Pyramid((0, 0, 0), E, F, G) → d, face1, edgeFG, face2, faceEFG, face3, edge1, edgeEF, edge2, edge3, edgeEG
  12. Textfield(A) → InputBox1
  13. Textfield(B) → InputBox2
  14. Textfield(C) → InputBox3

AI Drawing Prompt Reference

Draw a 3D coordinate system with an ellipsoid whose three semi-axis lengths are adjustable via input boxes. Pick a point on the ellipsoid surface and construct the tangent plane at that point. This tangent plane intersects the three coordinate planes (xy, xz, yz) along three lines, which pairwise meet at three vertices. Together with the origin, these vertices form a tetrahedron. Display the tetrahedron with semi-transparent faces, highlight the tangency point on the ellipsoid, and show the tangent plane. As the ellipsoid parameters change, the tangent plane and tetrahedron update accordingly, allowing exploration of the minimum tetrahedron volume.
The prompt above was compiled from the original drawing commands and is for reference only.