18.汽车的速度V车=36km/h=10m/s听到回声时汽车向前行驶的路程S车=V车t=10t声音传播的路程S声=V声t=340t则S=(S车+S声）/2即2S=10t+340t350t=2*700t=4s听到回声时汽车离山崖S距=S-S车=700m-10m/s*4s=660m
