In geometry, a prismatoid is a polyhedron whose vertices all lie in two parallel planes. Its lateral faces can be trapezoids or triangles.[1] If both planes have the same number of vertices, and the lateral faces are either parallelograms or trapezoids, it is called a prismoid.[2]

Prismatoid with parallel faces A1 and A3, midway cross-section A2, and height h

Volume edit

If the areas of the two parallel faces are A1 and A3, the cross-sectional area of the intersection of the prismatoid with a plane midway between the two parallel faces is A2, and the height (the distance between the two parallel faces) is h, then the volume of the prismatoid is given by[3]

This formula follows immediately by integrating the area parallel to the two planes of vertices by Simpson's rule, since that rule is exact for integration of polynomials of degree up to 3, and in this case the area is at most a quadratic function in the height.

Prismatoid families edit

Pyramids Wedges Parallelepipeds Prisms Antiprisms Cupolae Frusta

Families of prismatoids include:

Higher dimensions edit

A tetrahedral-cuboctahedral cupola.

In general, a polytope is prismatoidal if its vertices exist in two hyperplanes. For example, in four dimensions, two polyhedra can be placed in two parallel 3-spaces, and connected with polyhedral sides.

References edit

  1. ^ Kern, William F.; Bland, James R. (1938). Solid Mensuration with proofs. p. 75.
  2. ^ Alsina, Claudi; Nelsen, Roger B. (2015). A Mathematical Space Odyssey: Solid Geometry in the 21st Century. The Mathematical Association of America. p. 85. ISBN 9780883853580.
  3. ^ Meserve, B. E.; Pingry, R. E. (1952). "Some Notes on the Prismoidal Formula". The Mathematics Teacher. 45 (4): 257–263. JSTOR 27954012.

External links edit