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标题: [求助]PP1-PS-142 與 DS-50 [打印本页]

作者: idahuang 时间: 2008-2-19 21:57
标题: [求助]PP1-PS-142 與 DS-50

[PS-142] The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A present and inversely proportional to the concentration of chemical B present. If the concentration of chemical B is increased by 100 percent, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

(A) 100% decrease
(B) 50% decrease
(C) 40% decrease
(D) 40% increase
(E) 50% increase

答案是D

[DS-50] A construction company was paid a total of $500,000 for a construction project. The company's only costs for the project were for labor and materials. Was the company's profit for the project greater than $150,000 ?
(1) The company's total cost was three times its cost for materials.
(2) The company's profit was greater than its cost for labor.
答案是C ，但我一直以为是E

这两道题我想了很久都不知道该怎么算，拜托来个好心的MM帮我解惑阿~再过两周就要考试了

[此贴子已经被作者于2008-2-19 22:17:05编辑过]

作者: ciscogeek 时间: 2008-2-19 22:01

0.5 * (1 + x)^2 = 1

x = 40%

作者: idahuang 时间: 2008-2-19 22:36

呜..能不能麻烦这位大大解释一下呢？还是无法理解，感激阿

作者: heslaw 时间: 2008-2-19 22:50

original rate= X

now since B increase in conc, and since inversely proportional to rate, so 0.5x

so now we need to bring A conc up, so that (A+change)^2 * 0.5X=X

Q2, CT= CM+CL

P>CL, and CT=3CM, so CL=2CM, so P>2CM

since Total $= CT+P= 500K

even if p=2CM, and since CT=3CM

we know that P must be at least 200K

作者: ciscogeek 时间: 2008-2-19 23:59

呵呵，这两题我都是昨天做的。但数学还是错了8个。楼上的兄弟很强的，问他好了，但估计不是MM

作者: heslaw 时间: 2008-2-20 00:03

以下是引用ciscogeek在2008-2-19 23:59:00的发言：

呵呵，这两题我都是昨天做的。但数学还是错了8个。楼上的兄弟很强的，问他好了，但估计不是MM

汗，我的确是个boy...

作者: idahuang 时间: 2008-2-20 20:56

谢谢ciscogeek和heslaw的帮忙，我了解了！谢谢你们

作者: sausau 时间: 2008-11-25 01:38
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