a) d₁=d₂
d₁=d₂=d/2
d₁+d₂=d
t₁+t₂=t
t₁=d₁/v₁=d/2v₁
t₂=d₂/v₂=d/2v₂
d=vm*t=>vm=d/(t₁+t₂)=d/(d/2v₁+d/2v₂)=1/(1/2v₁+1/2v₂)=1/(v₁+v₂)/2v₁v₂=2v₁v₂/(v₁+v₂)
vm=2v₁v₂/(v₁+v₂)
b)t₁=t₂
t₁=t₂=t/2
t₁+t₂=t
d₁+d₂=d
d₁=v₁t₁=v₁t/2
d₂=v₂t₂=v₂t/2
d=d₁+d₂=v₁t/2+v₂t/2=t(v₁+v₂)/2
d=vm*t=>vm=d/t=t(v₁+v₂)/2/t=(v₁+v₂)/2
vm=(v₁+v₂)/2