Twin Paradox, Lorentz Transformation, Relativity.
two speeds v(t) and v_1(t_1) are also to be considered as positive values, even the exact expression of the differential dx_1 is dx_1 = -v_1(t_1)*dt_1. In the demonstration shown dx_1 an infinitesimal distance (and therefore positive) has been considered, also pay attention to the signs of the