42,a tunnel is a semicircular(直径是20feet).A single lane is 12 feet, and the distance to the sides of the tunnel is the same. if all the vehicles passing the tunnel should leave at least 1/2 feet distance to the top of the tunnel. what is the max height of the vehicle allowed?
answer is 7.5.
不大理解题意，没看明白详解版提供的思路，请哪位高人指点一下！谢谢！ | 
