求NN，问几道prep数学，5/28 急急急

kevinh1217 · 发表于 2013-5-26 14:29:36

今天又摸了一边，还是有几道提不太会做，请求NN们指点，谢了！！

A boat travelled upstream a distance of 90 miles at an average speed of  (v-3) miles per hour and then travelled the same distance downstream at an average speed 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.?

A 2.5
B 2.4
C 2.3
D 2.2
E 2.1
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Six countries in a certain region sent a total of 75 representatives to an international congress, and no two countries sent the same number of representatives. Of the six countries, if country A sent the second greatest number of representatives, did country A send at least 10 representatives?
1.  One of the six countries sent 41 representatives to the congress
2. Country A sent fewer than 12  representatives to the congress

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Last month 15 homes were sold in Town X. The average(arithmetic mean) sale priceof the homes was $150,000 and the median sale price was $130,000. Which of the following statements must be true?
I  At least one of the homes was sold for  more than $165,000
II. At least one of the homes was sold for more than $130,000 and less than $150,000
III.  At least one of the homes was sold for less than $130,000

A. I only
B. II only
C. III only
D I and II
E  I and III
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If x is possible , which of the following could be the correct ordering of 1/x; 2x; and x^2?
I. x^2 <2x < 1/x
II. x^2 < 1/x <2x
III. 2X < x^2 < 1/x

A. none
B. I only
C. III only
D I and II only
E I ,II and III
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In the xy-plane, does the line with equation  y= 3x+2 contain the point  (r,s)?
1.  (3r+2-s)(4r+9-s)=0
2.  (4r-6-s)(3r+2-s)=0

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还有一道几何，在附件离了，求各路同行援手，谢谢了！

kevinh1217 · 发表于 2013-5-26 20:39:07

求各路神仙啊啊啊

kevinh1217 · 发表于 2013-5-27 07:45:17

。。。。。。。。。。

kevinh1217 · 发表于 2013-5-27 10:15:54

求支援。。

daisy_rain · 发表于 2014-8-20 18:52:42

第一题我也错了，找到了解决方法，虽然时间过了，希望可以帮到后面遇到这道题的同学
Think of an intuitive manner or:
Start by replacing (v-3) and (v+3) with r = rate for upstream trip and r+6 = rate for downstream trip. That gives you:
90 = r(t+0.5)
90 = (r+6)t
and these are much nicer to work with. Make sure to set the plain variable t for the downstream portion of the trip, since that's what the problem asks you to find.

90 = rt + 0.5r upstream
90 = rt + 6t downstream
subtract
0 = 0.5r - 6t

next, solve for r to substitute, not t, since we want to solve for t eventually
6t = 0.5r
12t = r

substitute into 2nd equation (downstream) b/c that one's easier
90 = 12t(t) + 6t
90 = 12t^2 + 6t
0 = 12t^2 + 6t - 90 simplify: divide by 6
0 = 2t^2 + t - 15
= (2t-5)(t+3)
t = either 2.5 or -3. And it can't equal a negative, so it must be 2.5.

求NN，问几道prep数学，5/28 急急急

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