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A **polyhedron** is a solid bounded by flat polygons, which are called **faces.** Sides of the faces are called **edges**, and their corners are called as **vertices**. Depending on the number of faces one can specify tetrahedron (4 faces), pentahedron (5 faces), etc. A polyhedron is considered to be **convex** if it is located on one side of the plane of each of its faces. A polyhedron is called **regular** if its faces are regular polygons (i.e., where all sides and angles are equal) and all polyhedral angles at the vertices are equal. There can be found five types of regular polyhedra: tetrahedron, cube (regular hexahedron), octahedron, dodecahedron, icosahedron.

In a three-dimensional space a** polyhedron** *(the concept of a polyhedron)* is a set of finite number of flat polygons where

1) every side of a polygon is a side of the other one (but only one), called adjacent to the first one (along this side);

2) every polygon, which is a part of a polyhedron, can be reached by using another polygon adjacent to it, and this one in turn can be used to reach another adjacent polygon, etc.

These polygons are called **faces**, their sides are called **edges**, and their vertices are the **vertices** of the polyhedron.

A polyhedron is said to be **convex** if it is located on one side of the plane of each of its faces.

This definition implies that all faces of a convex polyhedron are flat convex polygons. Surface of a convex polyhedron consists of faces which lie on different planes. At the same time sides of polygons are edges of polyhedron, vertices of polyhedron are vertices of faces, plane angles of polyhedron are angles of polygons-faces.

A convex polyhedron is called a prismatoid if its vertices lie in two parallel planes. Prisms, pyramids and truncated pyramids are particular cases of a prismatoid. All lateral faces of a prismatoid are triangles or quadrangles, while quadrangular faces are trapezoids or parallelograms.

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