1.S1=V1*t1 S2=V2*t2
S总=S1+S2
v=S总/(t1+t2)=(V1*t1+V2*t2)/(t1+t2)
2 t1= x1/v1 t2=x2/v2
v=(x1=v2)/(x1/v1+x2/v2)