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.
There are no other shapes that have only four flat faces except for a tetrahedron, although the faces thatmake up the tetrahedron can be triangles of various shapes and need not be all identical.
Isosceles Tetrahedron.
Four triangular faces.
Sphere, cuboid, tetrahedron.
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.
In 2 dimensions, a quadrilateral. In 3 dimensions, a tetrahedron.
Just one: a tetrahedron or triangular pyramid.
A tetrahedron (4 faces) and an icosahedron (20).
Pentagon (and in 3d: triangular prism and tetrahedron.)
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.
There are many possible shapes. The regular ones amongst them are the tetrahedron, the octahedron and the icosahedron.