2015年05月31日

The stopping distance of a car is the number of feet that the car travels after the driver starts applying the brakes. The stopping distance of a certain car is directly proportional to the square of the speed of the car, in miles per hour, at the time the brakes are first applied. If the car’s stopping distance for an initial speed of 20 miles per hour is 17 feet, what is its stopping distance for an initial speed of 40 miles per hour?

A.34 feet

B.51 feet

C.60 feet

D.68 feet

E.85 feet

正确答案:D

解析: The stopping distance is directly proportional to the square of the initial speed of the car. If           represents the initial speed of the car, in miles per hour, and             represents the stopping distance, you have that the stopping distance is a function of           and that              , where            is a constant. Since the car’s stopping distance is              feet for an initial speed of  miles per hour, you know that . Therefore, , and the car's stopping distance for an initial speed of  miles per hour is  feet.