Bifrustum

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An n-gonal bifrustum is a convex polyhedron made from three parallel planes of n-gons. The middle plane is the largest, and the top and bottom planes are congruent. It can be formed by joining two congruent frusta along a symmetry plane, or by truncating the two ends of a bipyramid. It is the dual of the family of elongated bipyramids; three bifrusta are dual to Johnson solids J14–J16.

Structure
- Faces: 2 n-gons and 2n trapezoids
- Edges: 5n
- Vertices: 3n
- Symmetry: Dnh

Regular n-gonal bifrustum (parameters)
- Equatorial edge length: a
- Base edge length: b
- Half-height (distance from the middle plane to a base plane): h

Lateral surface area, total surface area, and volume
- Lateral area: Al = n (a + b) sqrt( ((a − b)/2 · cot(π/n))^2 + h^2 )
- Total area: A = Al + n b^2 / (2 tan(π/n))
- Volume: V = n (a^2 + b^2 + a b) h / (6 tan(π/n))

Notes
- The volume V is twice the volume of a single frustum.
- In general, an n-gonal bifrustum has 2n trapezoids and 2 n-gons, and is dual to the elongated dipyramids.