[求助]明天打G,两道题不懂...

issac711

1: 9.4  9.9  9.9  9.9  10.0  10.2  10.2  10.5 

The mean and the standard deviation of the 8 numbers shown are 10.0 and 0.3, respectively. what percent of the 8 numbers are within 1 standard deviation of the mean?

90%

85%

80%

75%

70%

2: 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 present, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

100% increase

50% increase

40% increase

40% decrease

50% decrease

谢谢各位XDJM

brightcoolsz

1.75%

2.2= (1+X)^2  X=40%  

hebeqiao

1 “1 standard deviation of the mean“ is 10.0-0.3=9.7 and 10.0+0.3=10.3, so the question is asking the persent of numbers that bigger than 9.7 and smaller than 10.3. Altogether 6 numbers are qualified: 9.9  9.9  9.9  10.0  10.2  10.2 . There are 8 numbers in the list. So the persent is 6/8=75%

2 Since "the rate of a certain chemical reaction is inversely proportional to the concentration of chemical B present and is directly proportional to the square of the concentration of chemical A present ",  the concentration of chemical A needs to be increased to offset the increase of concentration of chemical B in order to keep the reaction rate unchanged.

B is increased to 2B, so  A^2 needs to be 2A^2, A needs to be the multiplication of A and the square root of 2. Since the square root of 2 is 1.414, A need to be increased by 41.4% to be 1.414A. The answer is 40%, which is close to 41.4%.

东岸老男人

呵呵，是prep里面的吧？

其实搞懂概念了都很简单。

第一个，去画图理解什么叫mean和standard deviation ,1个SD,就是说mean左右的一个SD，这里SD是0.3，所以范围就是10-0.3到10+0.3. 你再看8个数里面一共有几个数是在这个范围里面的，6个，所以比例是6/8=0.75

第二个看懂了也简单：就是理解为一个函数y=a^2/(b), 现在b的数量上升1倍，变成2b，问你a^2怎么变，y值保持不变：很简单吧。就是也增加根号2倍就可以抵消b的变化。

根号2大概是1.41。所以大约增加40%。

不对的请其他同学指出！谢谢

[此贴子已经被作者于2008-2-9 5:51:51编辑过]

issac711

谢谢各位了!