解: v^2=kLd 60^2=9kl^2 kL^2=400 k=400/L^2 d=v^2/kL Q=v/(d+L)=v/(v^2/(kL)+L) =1/[v/(KL)+L/v] 分母=v/(KL)+L/v>=2根号[v/(kL)*L/v]=L/10 此时Q最大=10/L
