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decahedron
a polyhedron * * * * * That is only if it is bounded by plane faces. A sphere, for example, is a three dimensional shape but is not a polyhedron: it is a solid shape.
tetrahedron, a triangular pyramid
A di-tetrahedron.
tetrahedron
A two-dimensional shape with three sides is a triangle. A three-dimensional shape with three faces is a cylinder.
Two faces of a polyhedron form a wedge-shaped segment in three dimensional space.
It is a three-dimensional shape with twelve faces.
An edge.
A three dimensional shape bounded by plane (flat) faces is a polyhedron.
Pyramid
decahedron
a polyhedron * * * * * That is only if it is bounded by plane faces. A sphere, for example, is a three dimensional shape but is not a polyhedron: it is a solid shape.
A tetrahedron, also called a pyramid is a three-dimensional shape with four faces.
The three-dimensional platonic solid with eight faces is an octahedron.
tetrahedron, a triangular pyramid
A di-tetrahedron.