Hyperbolic triangle in the special theory of relativity
  The vector form of a Lorentz transformation which is separated with time and space parts is studied. It is necessary to introduce a new definition of the relative velocity in this transformation, which plays an important role for the calculations of various invariant physical quantities. The Lorentz transformation expressed with this vector form is geometrically well interpreted in a hyperbolic space. It is shown that the Lorentz transformation can be interpreted as the law of cosines and sines for a hyperbolic triangle in hyperbolic trigonometry. So the triangle made by the two origins of inertial frames and a moving particle has the angles whose sum is less than $180 ^o$. 
Yongkyu Ko