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Summary Article: tetrahedron
from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

In geometry, a solid figure (polyhedron) with four triangular faces; that is, a pyramid on a triangular base. A regular tetrahedron has equilateral triangles as its faces.

The volume (V) of a tetrahedron is given by

V = 1/3Bh

where B is the area of the base and h is the perpendicular height.

In chemistry and crystallography, tetrahedra describe the shapes of some molecules and crystals; for example, the carbon atoms in a crystal of diamond are arranged in space as a set of interconnected regular tetrahedra.

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tetrahedron

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Full text Article tetrahedron (plural tetrahedra)
The Penguin Dictionary of Mathematics

A solid figure that has four triangular faces (i.e. a triangular pyramid). A regular tetrahedron, in which the faces are equilateral triangles,...

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