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# Polyhedron. What is it?

A polyhedron is a solid bounded by flat polygons, which are called faces. The sides of the faces are called edges, and their corners are called vertices. Depending on the number of faces one can specify a 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 are 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. polyhedron vertices polyhedron edges polyhedron faces

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. The surface of a convex polyhedron consists of faces that lie on different planes. At the same time, the sides of polygons are the edges of polyhedron, the vertices of the polyhedron are the vertices of its faces, and the plane angles of the polyhedron are the angles of its 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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