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# Straight-Edged Objects Help

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By — McGraw-Hill Professional
Updated on Oct 25, 2011

## Introduction to Straight-Edged Objects

In Euclidean three-space, geometric solids with straight edges have flat faces, also called facets, each of which forms a plane polygon. An object of this sort is known as a polyhedron .

## The Tetrahedron

A polyhedron in 3D must have at least four faces. A four-faced polyhedron is called a tetrahedron . Each of the four faces of a tetrahedron is a triangle. There are four vertices. Any four specific points, not all in a single plane, form a unique tetrahedron.

### Surface Area of Tetrahedron

Figure 10-7 shows a tetrahedron. The surface area is found by adding up the interior areas of all four triangular faces. In the case of a regular tetrahedron, all six edges have the same length, and therefore all four faces are equilateral triangles. If the length of each edge of a regular tetrahedron is equal to s units, then the surface area, B , of the whole four-faced regular tetrahedron is given by:

B = 3 1/2 s 2

In this formula, 3 1/2 represents the square root of 3, or approximately 1.732.

Fig 10-7. A tetrahedron has four faces (including the base) and six edges.

### Volume of Tetrahedron

Imagine a tetrahedron whose base is a triangle with area A , and whose height is h as shown in Fig. 10-7. The volume, V , of the figure is given by:

V = Ah/3

## Pyramid

Figure 10-8 illustrates a pyramid . This figure has a square or rectangular base and four slanted faces. If the base is a square, and if the apex (the top of the pyramid) lies directly above a point at the center of the base, then the figure is a regular pyramid, and all of the slanted faces are isosceles triangles.

Fig 10-8. A pyramid has five faces (including the base) and eight edges.

### Surface Area of Pyramid

The surface area of a pyramid is found by adding up the areas of all five of its faces (the four slanted faces plus the base). In the case of a regular pyramid where the length of each slanted edge, called the slant height, is equal to s units and the length of each edge of the base is equal to t units, the surface area, B , is given by:

B = t 2 + 2 t ( s 2t 2 /4) 1/2

In the case of an irregular pyramid, the problem of finding the surface area is more complicated, because it involves individually calculating the area of the base and each slanted face, and then adding all the areas up.

### Volume of Pyramid

Imagine a pyramid whose base is a square with area A , and whose height is h as shown in Fig. 10-8. The volume, V , of the pyramid is given by:

V = Ah /3

This is true whether the pyramid is regular or irregular.

## The Cube

Figure 10-9 illustrates a cube . This is a regular hexahedron (six-sided polyhedron). It has 12 edges, each of which is of the same length, and eight vertices. Each of the six faces is a square.

### Surface Area of Cube

Imagine a cube whose edges each have length s , as shown in Fig. 10-9. The surface area, A , of the cube is given by:

A = 6 s 2

Fig 10-9. A cube has six square faces and 12 edges of identical length.

### Volume of Cube

Imagine a cube as defined above and in Fig. 10-9. The volume, V , of the solid enclosed by the cube is given by:

V = s 3

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