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A cube is a special case of rectangular cuboid in which the edges are equal in length. [1] Like other cuboids, every face of a cube has four vertices, each of which connects with three congruent lines. These edges form square faces, making the dihedral angle of a cube between every two adjacent squares being the interior angle of a square, 90 ...
The two skew perpendicular opposite edges of a regular tetrahedron define a set of parallel planes. When one of these planes intersects the tetrahedron the resulting cross section is a rectangle. [11] When the intersecting plane is near one of the edges the rectangle is long and skinny. When halfway between the two edges the intersection is a ...
The 24-cell can be constructed from 96 equilateral triangles of edge length √ 2, where the three vertices of each triangle are located 90° = π / 2 away from each other on the 3-sphere. They form 48 √ 2 -edge tetrahedra (the cells of the three 16-cells ), centered at the 24 mid-edge-radii of the 24-cell.
Doubling the cube is the construction, using only a straightedge and compass, of the edge of a cube that has twice the volume of a cube with a given edge. This is impossible because the cube root of 2, though algebraic, cannot be computed from integers by addition, subtraction, multiplication, division, and taking square roots.
Shallow truncation - Edges are reduced in length, faces are truncated to have twice as many sides, while new facets are formed, centered at the old vertices. Uniform truncation are a special case of this with equal edge lengths. The truncated cube, t{4,3}, with square faces becoming octagons, with new triangular faces are the vertices.
The two-fold axes give rise to three D 2h subgroups. T h is isomorphic to A 4 × Z 2 (since T and C i are both normal subgroups), and not to the symmetric group S 4. It is the symmetry of a cube with on each face a line segment dividing the face into two equal rectangles, such that the line segments of adjacent faces do not meet at the edge.
Conversely, any quadrilateral in which a 2 + c 2 = b 2 + d 2 must be orthodiagonal. [5] This can be proved in a number of ways, including using the law of cosines, vectors, an indirect proof, and complex numbers. [6] The diagonals of a convex quadrilateral are perpendicular if and only if the two bimedians have equal length. [6]
When obtuse triangles are glued in this way, the resulting surface can be folded to form a disphenoid (by Alexandrov's uniqueness theorem) but one with acute triangle faces and with edges that in general do not lie along the edges of the given obtuse triangles. Two more types of tetrahedron generalize the disphenoid and have similar names.