It is a solution to the problem, but expressed by the parameters of velocity. Return substitution to get the solution for velocities is: v 1 / c = tanh ⁡ ⁡ ( s 1 ) = e s 1 − − e − − s 1 e s 1 + e − − s 1 v 2 / c = tanh ⁡ ⁡ ( s 2 ) = e s 2 − − e − − s 2 e s 2 + e − − s 2