By analogy with complex numbers, a system of hyperbolic numbers can be
introduced in the same way: z=x+h*y with h*h=1 and x,y real numbers. As complex
numbers are linked to the Euclidean geometry, so this system of numbers is
linked to the pseudo-Euclidean plane geometry (space-time geometry). In this
paper we will show how this system of numbers allows, by means of a Cartesian
representation, an operative definition of hyperbolic functions using the
invariance respect to special relativity Lorentz group. From this definition,
by using elementary mathematics and an Euclidean approach, it is
straightforward to formalize the pseudo-Euclidean trigonometry in the Cartesian
plane with the same coherence as the Euclidean trigonometry.