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主题：[The Official SAT Question of the Day™] January 24th Saturday

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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

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SAT题目：http://apps.collegeboard.com/qotd/question.do

楼主 Date: 2009-01-24 13:02:46

jasontomonaga

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Hint : The stopping distance is directly proportional to the square of the initial speed of the car. If s represents the initial speed of the car, in miles per hour, and d represents the stopping distance, you have that the stopping distance is a function of s and that d(s) = cs2, where c is a constant.

沙发 Date: 2009-01-24 13:03:05

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Correct Answer: D

Here's Why: The stopping distance is directly proportional to the square of the initial speed of the car. If s represents the initial speed of the car, in miles per hour, and d represents the stopping distance, you have that the stopping distance is a function of s and that
d(s) = cs2, where c is a constant. Since the car’s stopping distance is 17 feet for an initial speed of 20 miles per hour, you know that 17 = c(202). Therefore,
c = = 0.0425, and the car's stopping distance for an initial speed of 40 miles per hour is (0.0425)(402) = 68 feet.
Difficulty: Medium
Question Type: Standard Multiple Choice
(Mathematics)

板凳 Date: 2009-01-24 13:03:30