\frac{\frac{v_1+\Delta v}{1+v_1\Delta v/c^2}+\frac{v_2-\Delta v}{1-v_2\Delta v/c^2}}{1+\frac{v_1+\Delta v}{1+v_1\Delta v/c^2}\frac{v_2-\Delta v}{1-v_2\Delta v/c^2}/c^2}=\frac{v_1+v_2}{1+v_1v_2/c^2}