Twin Paradox, Lorentz Transformation, Relativity.
in motion at speed v_1(t_1) and, in the frame S_1, it is less than dx and measures dx/γ (t_1). From dx_1=v_1(t_1)*dt_1 and from dx_1 = dx/γ(t_1) we get dx = γ(t_1)*v_1 (t_1)*dt_1. From dx = γ (t_1)*v_1(t_1)*dt_1 we then pass to v(t)*dt = γ(t_1)*v_1(t_1)*dt_1; finally it is possible to obtain the