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标题: 急求数学题解法—PREP [打印本页]

作者: blfichen 时间: 2009-12-26 15:39
标题: 急求数学题解法—PREP
1.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 B is increasedly by 100%, which is the closest to the percentage change in the concentration of A to keep the reaction rate unchanged.

Answer: 40% increased

2. The residents of town X paricipated in a survey to determine the hours per week each one spent watching TV. The result: mean = 21 hrs, standard deviation = 6 hrs, Pat(人名）was between 1 and 2 deviation below mean, how much time did he spent

Answer:12 hrs

不太明白什么意思。。希望NN解答下

作者: 驴在飞 时间: 2009-12-26 16:03
化学反应率搞不清楚状况，大概解法就是B变成2倍后A增加多少能跟原来的A平方比B成正比。1.4的平方为1.96，近似相等吧

作者: blfichen 时间: 2009-12-26 16:05

化学反应率搞不清楚状况，大概解法就是B变成2倍后A增加多少能跟原来的A平方比B成正比。1.4的平方为1.96，近似相等吧

-- by 会员驴在飞 (2009/12/26 16:03:14)

呀。。。是的呢。题意就没搞懂

作者: hzgary0010 时间: 2009-12-26 16:08
2. mean=21 sd=6 1-2 sd below mean 就是(21-2*6,21-6)====(9,15) 答案应该在这个区间内.

作者: hzgary0010 时间: 2009-12-26 16:13
1. 某化学反应和A的平方成正比和B成反比. 现在B增加100% 问A该如何变化.等式如下

A^2/B = (KA)^2/2B

得到 (KA)^2=2

K=1.414 所以选40%那个

作者: blfichen 时间: 2009-12-26 16:22

2. mean=21 sd=6 1-2 sd below mean 就是(21-2*6,21-6)====(9,15) 答案应该在这个区间内.

-- by 会员 hzgary0010 (2009/12/26 16:08:44)

感谢！不过还是不太明白2*6的原理= =

作者: blfichen 时间: 2009-12-26 16:24

1. 某化学反应和A的平方成正比和B成反比. 现在B增加100% 问A该如何变化.等式如下

A^2/B = (KA)^2/2B

得到 (KA)^2=2

K=1.414 所以选40%那个

-- by 会员 hzgary0010 (2009/12/26 16:13:01)

非常感谢。。再问个弱弱的问题：inversely proportional 就是反比，那如果是正负的比例呢？negative corelation?

作者: greekhero 时间: 2009-12-26 22:48

2. mean=21 sd=6 1-2 sd below mean 就是(21-2*6,21-6)====(9,15) 答案应该在这个区间内.

-- by 会员 hzgary0010 (2009/12/26 16:08:44)

感谢！不过还是不太明白2*6的原理= =

-- by 会员 blfichen (2009/12/26 16:22:12)

when you see a problem like this one, the key is to realize that you are really NOT being tested on anything that has to do with the actual meaning of the standard deviation. instead (and ironically), you should basically try to ignore the significance of the term 'standard deviation' entirely in this problem. for instance, let's say i gave you the following:
jimmy ate 33 hot dogs in an eating contest, and there are 8 hot dogs in a pack. if wes ate between 1 and 2 packs fewer than jimmy did, then how many hot dogs could wes have eaten?
you'd almost certainly have very little trouble with this version: the answer is between 33 - 2(8) and 33 - 8, or between 17 and 25.
same thing here: the answer is between 21 - 2(6) and 21 - 6, or between 9 and 15.

作者: blfichen 时间: 2009-12-27 14:15

2. mean=21 sd=6 1-2 sd below mean 就是(21-2*6,21-6)====(9,15) 答案应该在这个区间内.

-- by 会员 hzgary0010 (2009/12/26 16:08:44)

感谢！不过还是不太明白2*6的原理= =

-- by 会员 blfichen (2009/12/26 16:22:12)

when you see a problem like this one, the key is to realize that you are really NOT being tested on anything that has to do with the actual meaning of the standard deviation. instead (and ironically), you should basically try to ignore the significance of the term 'standard deviation' entirely in this problem. for instance, let's say i gave you the following:
jimmy ate 33 hot dogs in an eating contest, and there are 8 hot dogs in a pack. if wes ate between 1 and 2 packs fewer than jimmy did, then how many hot dogs could wes have eaten?
you'd almost certainly have very little trouble with this version: the answer is between 33 - 2(8) and 33 - 8, or between 17 and 25.
same thing here: the answer is between 21 - 2(6) and 21 - 6, or between 9 and 15.

-- by 会员 greekhero (2009/12/26 22:48:00)

Thx~

作者: 阿喵达达 时间: 2009-12-27 19:45

1. 某化学反应和A的平方成正比和B成反比. 现在B增加100% 问A该如何变化.等式如下

A^2/B = (KA)^2/2B

得到 (KA)^2=2

K=1.414 所以选40%那个

-- by 会员 hzgary0010 (2009/12/26 16:13:01)

非常感谢。。再问个弱弱的问题：inversely proportional 就是反比，那如果是正负的比例呢？negative corelation?

-- by 会员 blfichen (2009/12/26 16:24:08)

be directly proportional 成正比~~

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