美国数学竞赛AMC8真题2014年 25

美国数学竞赛AMC8专为8年级及以下的初中学生设计，但近年来的数据显示，越来越多小学4-6年级的考生加入到美国数学竞赛AMC8级别的考试行列中，而当这些学生能在成绩中取得“A”类标签，则是对孩子数学天赋的优势证明，不管是对美高申请还是今后在数学领域的发展都极其有利！那么接下来跟随小编来看一下美国数学竞赛AMC8的官方真题以及官方解答吧：

Problem 25

A straight one-mile stretch of highway, feet wide, is closed. Robert rides his bike on a path composed of semicircles as shown. If he rides at miles per hour, how many hours will it take to cover the one-mile stretch?

Note: mile = feet

Solution

There are two possible interpretations of the problem: that the road as a whole is feet wide, or that each lane is  feet wide. Both interpretations will arrive at the same result.  However, let us stick with the first interpretation for simplicity. Each  lane must then be feet wide, so Robert must be riding his bike in semicircles with radius feet and diameter feet. Since the road is feet long, over the whole mile, Robert rides semicircles in total. Were the semicircles full circles, their circumference would be feet; as it is, the circumference of each is half that, or feet. Therefore, over the stretch of highway, Robert rides a total of feet, equivalent to miles. Robert rides at 5 miles per hour, so divide the miles by mph (because t = d/r time = distance/rate) to arrive at hours.

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