Observe the five colourful objects that head my blog.
Leftmost is a tetrahedron — a pyramid with a triangular base.
Notice how regular and symmetric it is. It has four corners — mathematicians call them vertices (singular vertex) — and each vertex is host to exactly three edges. Each vertex is a corner of exactly three of the four faces of the tetrahedron. And each face has exactly three edges. There are six edges all together.
This regularity means that you get the same picture of the tetrahedron no matter which vertex is on top.
Compare that with the the standard Egyptian pyramid (or square pyramid), which has a square base and four triangular sides. Four edges meet at the peak vertex of the Egyptian pyramid, but the four base vertices play host to only three edges. Pick up an Egyptian pyramid and turn it about: you get a different picture depending on which vertex is uppermost. The Egyptian pyramid is not regular.
Notice that regularity does not depend on the lengths of the edges or the area of the faces. We can shorten one of the edges to create a distorted tetrahedron that is still regular, although somewhat twisted. Regularity means each of the faces has the same number of sides and the same number of edges meet at each vertex.
If you feel particularly geeky, you might like to learn a new technical word that will lose you friends when you mention it at parties. Both the tetrahedron and the Egyptian pyramid are three-dimensional polytopes, objects with flat faces bounded by edges that meet in vertices. A cut diamond is a polytope; a beach ball is not.
A regular polytope is one in which each vertex has the same number of edges meeting there and each face is bounded by the same number of edges.
If you are like me, you will prefer to call polytopes “solids” — much more descriptive.
The tetrahedron is the simplest regular solid. The other pictures in my header show four others: the cube, the octahedron, the dodecahedron and the icosahedron.
We can use those last facts to compute the number of edges without counting them on the diagram or a model.
Each face has five edges and there are 12 faces. So there appear to be 5 x 12 = 60 edges. But we have counted each edge twice; once from the point of view of one face and once from the point of view of the adjacent face. So there are 60/2 = 30 edges.
Let’s use the same method of computing edges that we used for the dodecahedron. Three edges per face and 20 faces gives us an initial count of 3 x 20 = 60 edges. But each one has been counted twice. So there are 60/2 = 30 edges. Same as the dodecahedron.
We can also count edges from the point of view of the vertices. There are 12 vertices and five edges meet at each. An initial count of the edges is 12 x 5 = 60. But we have counted each edge twice — once at each end. So the number of edges is 60/2 = 30.
We’ve developed two rules for counting edges, rules that apply to all regular solids.
Let F be the number of faces, V be the number of vertices and E be the number of edges. Also suppose each face has m sides or edges to it and let n be the number of edges meeting at each vertex. (m and n are the same for each face or vertex exactly because we are talking about regular solids.) Then the number of edges can be expressed in two different ways.
A bit of high school algebra gives us the following.
These rules are true for all five of the regular solids.
Now — are they true for all regular solids? Can you think of any more regular solids? Remember each face has to have the same number of edges and each vertex must play host to the same number of edges.
More to come.