题目

阅读理解 People aren't walking any more—if they can figure out a way to avoid it. I felt superior about this matter until the other day I took my car to mail a small parcel. The journey is a matter of 281 steps. But I used the car. And I wasn't in any hurry, either. I had merely become one more victim of a national sickness: motorosis. It is an illness to which I had thought myself immune (免疫的) , for I was bred in the tradition of going to places on my own two legs. At that time, we regarded 25 miles as a good day's walk and the ability to cover such a distance in ten hours as a si某落地钟钟摆长为0.5m，来回摆到最大，夹角为20°，已知在钟摆的摆动过程中，摆锤离地面的最低高度为am，最高高度为bm，则b-a=    m．（结果精确到0.0001，参考数据cos10°=0.985）