请教高手们几道prep数学题

narsa

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 respresent. If the concentration of chemical B is incresed by 100 percent, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction reate unchanged?
100% decrease
50% decrease
40% decrease
40% increase
50 increase

2. For every integer k from 1 to 10, inclusive, the kth term of a certain sequence is given by (-1)^k+1(1/2^2),就是-1的k+1次方乘以2的k次方分之1. If T is the sum of the first 10 terms in sequence, then T is ?

3. Town x is doing a survey about TV watching. The distrubution of the results of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of hours that Pat, a resident of Town X, watched TV last week was between 1 and 2 standard deviations below the mean. What is the number of hours Pat watched TV last week?
30
18
12
6
20
谁能给我讲讲这个standard deviation啊？我就知道它是什么，但是一出来就又茫然了。
非常感谢大家了！

sdcar2010

Google within this forum.  All of your questions have been discussed before.

gmat2000

1.題目意思是說 化學物A的平方與此化學反應速率成正比
化學物B 與此化學反應速率成反比
所以B增加100% 換句話說B變成了兩倍 所以反應速率變成1/2
要讓原本的速率不變 所以A勢必要增加 能讓速度變成兩倍去抵銷那1/2倍
得到算式會是2*1/2=1
A的平方與速率成正比 所以A平方就等於2 A就等於根號2=1.41...

yanyan7910

3，The number of hours that Pat, a resident of Town X, watched TV last week was between 1 and 2 standard deviations below the mean.
你只要读懂这句话就可以列出式子。并不是要你用标准方差的计算式去计算。式中的mean 和standard deviation都是迷惑你。如果换成mean 为a和standard deviation 为b, 还是一样的计算方法：x=mean-1倍standard deviations或x=mean-2倍standard deviation.

yanyan7910

1, 只要你根据第一句话列出式子：R=K（A平方）/B，其中K为常数，就可以解出题了

yanyan7910

2，For any integer k from 1 to 10, inclusive, the kth of a certain sequence is given by [(-1)^(k+1)]*(1 / 2^k). If T is the sum of the first 10 terms of the sequence, then T is:
当K=1，A1=1/2，T1=1/2
当K=2，A2=-（1/4），T2=1/2-1/4
当K=3，A3=1/8，T3=1/2-1/4+1/8
当K=4，A4=-（1/16），T4=1/2-1/4+1/8-1/16，
依次类推，T10=1/2-1/4+1/8-1/16+...+1/2^9-1/4^5,
你可以看出等比数列，计算一下就得出T10=1/3了

narsa

大家都太好了，感激涕零啊！感觉这么一说挺简单的么，自己做的时候就头大了