The answer depends on ... compared to WHAT?
A tetrahedron is a pyramid with a triangular base. An example would be this kite: http://en.wikipedia.org/wiki/Tetrahedral_kite
James used his tetrahedron today.
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.
I am not a tetrahedron!
A tetrahedron has 4 faces.
Well, a tetrahedron has four sides. A regular tetrahedron is a triangular pyramid.
A tetrahedron must be plane faced and so, it is just an ordinary tetrahedron.
For entertainment - to fly it
He faced a tetrahedron question in the exam paper. This is a sample sentence using the word tetrahedron.
A Tetrahedron. The most common form of a tetrahedron is called a regular tetrahedron, or a pyramid
A tetrahedron has 4 faces.