v3=s/t t=t1+t2
v3=s/(t1+t2)=1/(t1/s+t2/s)
v1=(s/2)/t1 t1/s=1/2v1
v2=(s/2)/t2 t2/s=1/2v2
v3=1/(1/2v1+1/2v2)=2v1v2/(v1+v2)