A bus covers the first half of a certain distance with speed v1 and the second half with a speed v2. The average speed during the whole journey is :
1
\(\frac{2 v_{1} v_{2}}{v_{1}+v_{2}} \)
2
\( \frac{v_{1}+v_{2}}{2} \)
3
\(\frac{v_{1} v_{2}}{v_{1}+v_{2}} \)
4
\(\sqrt{v_{1} v_{2}}\)