A tetrahedron is a four sided polyhedron. Each face is a triangle. If it is a regular tetrahedron, then each face is an equilateral triangle.
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Four triangular faces.
There can be no formula for a tetrahedron since you have not specified what aspect the formula is supposed to calculate. Furthermore, although they are all triangular pyramids, tetrahedra can be tall, needle-like shapes or almost flat shapes.
Pentagon (and in 3d: triangular prism and tetrahedron.)
A tetrahedron (4 faces) and an icosahedron (20).
A tetrahedron is the smallest possible polyhedron: a closed 3-d shape with polygonal faces. It has 4 triangular faces, 4 vertices and 6 edges.