问两道PREP题

bioliwu

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！

elitist

1.LZ的算式是正确的吧。

bioliwu

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？

himba

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……