properties of regular tetrahedron∗
pahio†
2013-03-22 1:55:44
A regular tetrahedron may be formed such that each of its edges is a diagonal
of a face of a cube; then the tetrahedron has been inscribed in the cube.
It’s apparent that a plane passing through the midpoints of three parallel
edges of the cube cuts the regular tetrahedron into two congruent pentahedrons
and that the intersection figure is a square, the midpoint M of which is the
centroid of the tetrahedron.
The angles between the four half-lines
√ from the centroid M of the regular
tetrahedron to the vertices are 2 arctan 2 (≈ 109◦ ), which is equal the angle
between the four covalent bonds of a carbon atom. A half of this angle, α, can
be found from the right triangle in the below figure, where the catheti are √s2
and 2s .
∗ hPropertiesOfRegularTetrahedroni created: h2013-03-2i by: hpahioi version: h41178i
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1
.
√s
2
M
s
α
s
s
.
One can consider the regular tetrahedron as a cone. Let its edge be a and its
height h. Because of symmetry, a height line intersects the corresponding base
triangle in the centroid of this equilateral triangle. Thus we have (see the below
diagram) the rectangular
triangle with hypotenuse√ a, one cathetus h and the
√
2 a 3
a
√
other cathetus 3 · 2 = 3 (i.e. 23 of the median a 2 3 of the equilateral triangle
— see the common point of triangle medians). The Pythagorean theorem then
gives
s
√
2
a
a 6
2
h =
a − √
=
.
3
3
.
a
h
a
2
a
2
.
√
a 6
Consequently, the height of the regular tetrahedron is
.
3
√
2
Since the area of the base triangle is a 4 3 , the volume (one third of the
√
a3 2
product of the base and the height) of the regular tetrahedron is
.
12
2