Platonic solid
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A Platonic solid is a polyhedron, or 3 dimensional figure, in which all faces are congruent regular polygons such that the same number of faces meet at each vertex. There are five such solids: the cube (regular hexahedron), the regular tetrahedron, the regular octahedron, the regular dodecahedron, and the regular icosahedron.
The cube has six faces, all of which are squares. It also has eight vertices and twelve edges. Three faces meet at each vertex.
The tetrahedron has four faces, all of which are triangles. It also has four vertices and six edges. Three faces meet at each vertex.
The octahedron has eight faces, all of which are triangles. It also has six vertices and twelve edges. Four faces meet at each vertex.
The dodecahedron has twelve faces, all of which are pentagons. It also has twenty vertices and thirty edges. Three faces meet at each vertex.
The icosahedron has twenty faces, all of which are triangles. It also has twelve vertices and thirty edges. Five faces meet at each vertex.
It is easy to verify that all five Platonic solids satisfy Euler's polyhedral formula.
Those are the only possible Platonic solids. In order to show that this is so, consider what polygons could be the faces. There must be at least 3 faces at each vertex. Thus, the faces must be polygons with fewer than 6 sides. Only three squares or pentagons can fit around a single vertex -- this gives rise to the cube and the dodecahedron. Anywhere from 3 to 5 triangles can fit around a single vertex, giving rise to the other three Platonic solids.
Duality
Interestingly enough, if you take a Platonic solid and connect the centers of adjacent faces with line segments, those segments will be the edges of another Platonic solid. This relationship is known as duality. It has many interesting properties. For instance: The dual of a tetrahedron is another tetrahedron. For this reason, we call the tetrahedron self-dual.
The cube and the octahedron are duals, so connecting the centers of the faces of a cube gives an octahedron and vice-versa. Cubes have the same number of faces as octahedra have vertices, and vice-versa. Cubes and octahedra have the same number of edges. Also, the number of faces at each vertex of a cube is the same as the number of edges on each face of an octahedron, and vice versa.
The icosahedron and the dodecahedron are duals, so connecting the centers of the faces of an icosahedron gives a dodecahedron and vice-versa. Icosahedra have the same number of faces as dodecahedra have vertices, and vice-versa. Icosahedra and dodecahedra have the same number of edges. Also, the number of faces at each vertex of an icosahedron is the same as the number of edges on each face of an dodecahedron, and vice versa.
Note that the number of edges of two duals is the same, and the number of vertices of one dual is the number of faces of the other dual. This article is a stub. Help us out by expanding it.
