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# Polyhedrons

## 3-D figures formed by polygons enclosing regions in space.

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Polyhedrons

### Polyhedrons

A polyhedron is a 3-dimensional figure that is formed by polygons that enclose a region in space. Each polygon in a polyhedron is a face. The line segment where two faces intersect is an edge. The point of intersection of two edges is a vertex.

Examples of polyhedrons include a cube, prism, or pyramid. Cones, spheres, and cylinders are not polyhedrons because they have surfaces that are not polygons. The following are more examples of polyhedrons:

The number of faces (F\begin{align*}F\end{align*}), vertices (V\begin{align*}V\end{align*}) and edges (E\begin{align*}E\end{align*}) are related in the same way for any polyhedron. Their relationship was discovered by the Swiss mathematician Leonhard Euler, and is called Euler’s Theorem.

Euler’s Theorem: F+V=E+2\begin{align*}F+V=E+2\end{align*}.

Faces+Vertices5+6=Edges+2=9+2\begin{align*}Faces + Vertices &= Edges +2\\ 5 + 6 &= 9 + 2\end{align*}

A regular polyhedron is a polyhedron where all the faces are congruent regular polygons. There are only five regular polyhedra, called the Platonic solids.

1. Regular Tetrahedron: A 4-faced polyhedron and all the faces are equilateral triangles.
2. Cube: A 6-faced polyhedron and all the faces are squares.
3. Regular Octahedron: An 8-faced polyhedron and all the faces are equilateral triangles.
4. Regular Dodecahedron: A 12-faced polyhedron and all the faces are regular pentagons.
5. Regular Icosahedron: A 20-faced polyhedron and all the faces are equilateral triangles.

What if you were given a solid three-dimensional figure, like a carton of ice cream? How could you determine how the faces, vertices, and edges of that figure are related?

### Examples

#### Example 1

Determine if the following solids are polyhedrons. If the solid is a polyhedron, name it and find the number of faces, edges and vertices its has.

The base is a triangle and all the sides are triangles, so this is a triangular pyramid, which is also known as a tetrahedron. There are 4 faces, 6 edges and 4 vertices.

This solid is also a polyhedron. The bases are both pentagons, so it is a pentagonal prism. There are 7 faces, 15 edges, and 10 vertices.

The bases are circles. Circles are not polygons, so it is not a polyhedron.

#### Example 2

In a six-faced polyhedron, there are 10 edges. How many vertices does the polyhedron have?

Solve for V\begin{align*}V\end{align*} in Euler’s Theorem.

F+V6+VV=E+2=10+2=6\begin{align*}F + V &= E + 2\\ 6 + V &= 10 + 2\\ V &= 6\end{align*}

Therefore, there are 6 vertices.

#### Example 3

Markus counts the edges, faces, and vertices of a polyhedron. He comes up with 10 vertices, 5 faces, and 12 edges. Did he make a mistake?

Plug all three numbers into Euler’s Theorem.

F+V5+1015=E+2=12+214\begin{align*}F + V &= E + 2\\ 5 + 10 &= 12 + 2\\ 15 & \neq 14\end{align*}

Because the two sides are not equal, Markus made a mistake.

#### Example 4

Find the number of faces, vertices, and edges in an octagonal prism.

There are 10 faces and 16 vertices. Use Euler’s Theorem, to solve for E\begin{align*}E\end{align*}.

F+V10+1624=E+2=E+2=E\begin{align*}F + V &= E + 2\\ 10 + 16 &= E + 2\\ 24 &= E\end{align*}

Therefore, there are 24 edges.

#### Example 5

A truncated icosahedron is a polyhedron with 12 regular pentagonal faces, 20 regular hexagonal faces, and 90 edges. This icosahedron closely resembles a soccer ball. How many vertices does it have? Explain your reasoning.

We can use Euler's Theorem to solve for the number of vertices.

F+V32+VV=E+2=90+2=60\begin{align*}F + V &= E + 2\\ 32 + V &= 90 + 2\\ V &= 60\end{align*}

Therefore, it has 60 vertices.

### Review

Complete the table using Euler’s Theorem.

Name Faces Edges Vertices
1. Rectangular Prism 6 12
2. Octagonal Pyramid 16 9
3. Regular Icosahedron 20 12
4. Cube 12 8
5. Triangular Pyramid 4 4
6. Octahedron 8 12
7. Heptagonal Prism 21 14
8. Triangular Prism 5 9

Determine if the following figures are polyhedra. If so, name the figure and find the number of faces, edges, and vertices.

To see the Review answers, open this PDF file and look for section 11.1.

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### Vocabulary Language: English Spanish

polyhedron

A 3-dimensional figure that is formed by polygons that enclose a region in space. Each polygon in a polyhedron is a face.

regular polyhedron

A polyhedron where all the faces are congruent regular polygons.

Polygon

A polygon is a simple closed figure with at least three straight sides.

Sphere

A sphere is a round, three-dimensional solid. All points on the surface of a sphere are equidistant from the center of the sphere.

Tetrahedron

A tetrahedron is a shape with a triangular base and sides, also correctly described as a triangular pyramid. There are 4 faces, 6 edges and 4 vertices in a regular tetrahedron.