Constructing a Platonic solid

Geometry Level 5

You have a collection of Platonic solids each having an edge length of one, Lets call these "unit Platonic solids". You realize that you can glue them all together to construct a much larger Platonic solid leaving no gaps or holes.

In your final construction (one large Platonic solid) you have used exactly 364 of one type of the unit Platonic solids, but may have used other types of unit Platonic solids as well.

How many unit Platonic solids did you use altogether in this final construction?

For example, if your final consturction is a Platonic solid mad up of 364 unit cubes and 212 unit tetrahedra, then your final answer would be 576.


Inspiration: Michael Mendrin

Image credit: www.greatlittleminds.com


The answer is 1105.

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1 solution

Geoff Pilling
Sep 11, 2016

There are five Platonic solids: Octahedron, Cube, Tetrahedron, Icosahedron, and Dodecahedron.

The only two Platonic solids that can be formed from different Platonic solids are the tetrahedron and the octahedron. (The cube is out, since 364 isn't a cubic number)

The formulae for constructing these solids (thanks @Michael Mendrin for providing these!) are as follows:

To make tetrahedra of linear size n n , you need

  • 1 6 n ( n 2 1 ) \frac{1}{6}n(n^2 - 1) octahedra
  • 1 3 n ( n 2 + 2 ) \frac{1}{3}n(n^2 + 2) tetrahedra

For example for n = 2 n=2 we have this construction:

To make octahedra of linear size n n , you need

  • 1 3 n ( 2 n 2 + 1 ) \frac{1}{3}n(2n^2 + 1) octahedra
  • 4 3 n ( n 2 1 ) \frac{4}{3}n(n^2 -1) tetrahedra

For example, for n = 2 n=2 , we have this construction:

For integer n n , the only equation which gives 364 364 of any of these is the first one.

This is for n = 13 n = 13 , which implies you have 364 364 ocathedra and 741 741 tetrahedra and you are constructing a large tetrahedron.

So the total number of Platonic solids you begin with is 364 + 741 = 1105 364+741=\boxed{1105}

Moreover, if all the faces of the tetrahedron and the octahedron are of the same size equilateral triangles, there's a number of interesting ways they can be combined to fill space. However, there's only one "honeycomb" of such polyhedra that can form either a regular tetrahedron or a regular octahedron, so that, for example, in a large honeycomb forming a tetrahedron, we can subdivide it into smaller tetrahedra and octahedra, each with their own such honeycombs.

Michael Mendrin - 4 years, 9 months ago

You don't need the specification that they need to be different as the 364 is not a cubic number.

Wen Z - 4 years, 9 months ago

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Well, that's right, that rules out at least the cube.

Michael Mendrin - 4 years, 9 months ago

Ah, yes... Redundant information... :-/ Good eyes! I've updated the problem and solution accordingly! :)

Geoff Pilling - 4 years, 9 months ago

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