The scalar product between two 3-vectors can be written ab= ab cos ab, where a and b are the magnitudes of the vectors and eab is the angle between them. show that an analogous formula holds for two timelike 4-vectors a, b: a b= -ab cosh (abs (2) where a = (-a. a)1/, b = (-6. b)/2 and vab = tanh oab, for vab being the lorentz boost between the frame in which an observer whose worldline points along a is at rest and the frame for which an observer whose worldline points along b is at rest.