GWD31-2 MATH求助！！！感激~~~~

dolorez · 发表于 2009-11-15 22:06:22

Q3:
If p, s, and t are positive prime numbers, what is the value of p3s3t3 ?
(1)    p3st = 728
(2)    t = 13

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.

Q18:
A certain characteristic in a large population has a distribution that is symmetric about the mean m. If 68 percent of the distribution lies within one standard deviation d of the mean, what percent of the distribution is less than m + d ?

A.   16%
B.   32%
C.   48%
D.   84%
E.   92%

Q32:
What is the probability that event E or event F or both will occur?
(1)    The probability that event E will occur is 0.6.
(2)    The probability that event F will occur is 0.4.

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.
Answer:

QX:
If the probability is 0.54 that Stock A will increase in value during the next month and the probability is 0.68 that Stock B will increase in value during the next month, what is the greatest possible value for the probability that neither of these two events will occur?

A.   0.22
B.   0.32
C.   0.37
D.   0.46
E.   0.63
Q36:
If x and y are integers, is y an even integer?
(1)    2y – x = x2 – y2
(2)    x is an odd integer.

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.

sayysong · 发表于 2009-11-15 22:26:04

#3 A
因数分解 p^3 * s * t = 728 = 2^3 * 13 * 7 则 p = 2, s * t = 13 * 7 = 91
p^3 * s^3 * t^3 = 728 * (s * t)^2 = 728 * 91^2

sayysong · 发表于 2009-11-15 22:27:05

#18 D
有人问过同样的问题，我回答过，你找找看。

sayysong · 发表于 2009-11-15 22:28:49

#32 E
因为E和F没有指出是什么关系，互斥还是独立

sayysong · 发表于 2009-11-15 22:34:13

#X B
要使两种可能性都不发生的概率最大。用Venn Diagram，一大一小两个圈圈，有交集，题目要求的就是两个圈圈以外的最大值。
那么小圈圈可以完全包含在大圈圈里面，这时他们的合集最小为0.68，取非为 1-0.68=0.32 就是答案了。

heavennancy · 发表于 2009-11-15 22:35:06

32 C 求两个事件至少有一个发生的概率，倒推需要知道这两个事件各自发生的概率吧。对吗

yangli317 · 发表于 2009-11-15 22:37:47

试着做了下：
Q3: (1) 728=2^3X7X13 那么p肯定是2，s和t不管谁是7或13，都已经可以得到p3s3t3了 (2) 只知道t是13条件不够
所以我觉得是A

Q18: 0.68在m-d和m+d之间，也就是说0.16小于m-d，0.16大于m+d, 要求多少小于m+d, 就是1－0.16=0.84 我觉得选D

Q32: 初步看来，要求得是至少一个发生得概率，就是1减两个都不发生概率。所以需要知道每个分别发生概率。所以选C吧。这题看似简单，怕有陷阱，也许我考虑有疏漏。

QX: 我觉得这题说道两者都不发生的最大可能的概率，应该是暗示要考虑这两个时间不是独立事件，而是highly correlated的事件。这里B有0.68会涨，0.32会跌；而A有0.54会涨，0.46会跌。如果两个不是独立事件，就应该是0.54概率一起涨，0.32概率一起跌，剩下0.14概率一涨一跌。所以我觉得都不发生的最大概率是0.32，选B。不知对不对。

Q36: (1)可以写成: y(y+2)=x(1+X) 右边一定是偶（奇乘偶），所以左边必须是偶乘偶，所以y是偶
(2)只看2条件不足
所以我觉得应该选A

希望能帮到你，我也当作自己练习一下。欢迎大家一起讨论，纠正我的错误

sayysong · 发表于 2009-11-15 22:45:24

#36 A
2y肯定为偶数，2y - x分奇偶两种情况讨论。右边的分解一下 x^2 - y^2 = (x+y) * (x-y)
情况1—— 2y - x为奇
2y-x = odd --> x = odd --> (x+y) = odd & (x-y) = odd --> y = even
情况2—— 2y - x为偶
2y-x = even --> x = even --> (x+y) = even || (x-y) = even --> y = even

dolorez · 发表于 2009-11-15 22:53:29

多谢解答。。。。。。明白了。这一题在CD中搜不到，，，能帮助解下麽？？谢谢。。

sayysong · 发表于 2009-11-15 22:57:33

多谢解答。。。。。。明白了。这一题在CD中搜不到，，，能帮助解下麽？？谢谢。。

-- by 会员 dolorez (2009/11/15 22:53:29)

第二题 D
A certain characteristic in a large population has adistribution that is symmetric about the mean m. If 68 percent of thedistribution lies within one standard deviation d of the mean, whatpercent of the distribution is less than m + d ?
题目说围绕平均值m做对称分布，有68%的分布在平均值周围(左右两边)标准方差为d的范围内，其实说的范围是 [m-d, m+d]。
现在要求的是[... , m+d]范围内的百分比。
画个数轴就知道了，分布在 [m-d, m+d]以外的是 1-68%=32%，左右各16%，再16%+68% = 84%。

http://forum.chasedream.com/GMAT_Math/thread-419301-1-1.html

GWD31-2 MATH求助！！！感激~~~~

GWD31-2 Q18

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