1. A boat travelede upstream a distance of 90 miles at an average spead of (V-3) miles per hour and then traveled the same distance downstream at an average of (V+3) miles per hour. If the trip upstream took half an hour longer than the trip downstream, how many hours did it take the boat to travel downstream?
answer: 2.5
我知道这题不难,问题是我怎么样都算不出这个答案。
2. For which following function is f(a+b)=f(a)+f(b) for all positive numbers a and b?
A. f(x)=X2
B. f(x)=X+1
C. f(x)=√X
D. f(x)=2/X
E. f(x)=-3X
Answer: E
请问解题思路。谢谢!
1。90/(v+3) - 90/(v-3)=0.5 得速度=33
所以t = 90/36=2.5
2。我说你可别打我哈,数学系的人一看通式就知道是一阶齐次方程。就是说一次的,没有常数项。不要问怎么看出来的,只是因为看多了。还有类似的很多抽象等式,都对应了一种方程。比如f(x+y)=f(xy)对应ln或者log……
谢谢你啊。
原文说 If the trip upstream took half an hour longer than the trip
downstream,所以应该是90/(V-3)-90/(V+3)=0.5啊!难道是我理解有误?
第二题我没你强啊,假如我看不出来,考试现场该如何处理?
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