A pyramid on a triangular base is called a tetrahedron. In other words, a tetrahedron is a shape bounded by four triangular faces. Obviously, a tetrahedron is a triangular pyramid. If the base of a tetrahedron is an equilateral triangle, and the other triangular faces are isosceles triangles, then it is called a right tetrahedron. A tetrahedron is considered regular if all four of its faces are equilateral triangles. It is clear that these equilateral triangles are congruent to each other..
Surface area of a tetrahedron