求助prep1的两道题（后天考试，大家帮个忙）

Shiry

真的不好意思开新帖，但是找了一圈也没翻到我想问的题，大概是太简单了都没有人问，如果有以前的链接麻烦发一个，因为后天考试有点赶了，麻烦会做的NN点拨下。数学错的快没信心考了……

PS:

n是5的multiple，同时n=p^2*q p、q都是prime,问n等于多少时是25的multiple?

1.p^2

2.q^2

3.pq

4.p^2*q^2

5.p^3*q

这个是一点思路都没有。

DS:

一个box里有三种球red.blue.white,上面有1-10的数字，随机抽球，问either white or even number 的概率

1.both white and even 的概率为0

2、white概率-even概率=0.2

答案是E   我选是C

streamflying

个人意见，仅供参考哈。

第一题：答案D.

n=p^2*q是5的倍数，p、q都prime，so p or q 一定有一个是5. 若n=p^2*q^2,则一定是25的multiple,选D。

第二题：根据2，even概率是0.5,so white概率是0.7，说明white有7个。

再看1，此时both white and even 的概率为0，不可能，因为如果要both white and even 的概率为0，white数目一定不能超过5。

mswlove

LZ mm，我认为如果第一题不知道怎么下手的话最简单就用特殊值法

Shiry

谢谢streamflying的解答哦 醍醐灌顶

mswlove的解法很适合考试当场用 谢谢哦

又翻了下prep 发现DS 的原题如下：

each of the 25 balls in a certain box is either red, blue or white and has a number from 1 to 10 painted on it. If one ball is to be selected at random from the box, what is the probability that the ball selected will either be white or have an even number painted on it?
1. the probability that the ball will both be white and have an even number painted on it is 0
2. the probability that the ball will be white minus the probability that the ball will have an even number painted on it is 0.2

这样就不能按streamflying的解法做了

我是这样想的

设white 的概率为a； even的概率为b

因为a-b=0.2  ab=0

所以(a+b)^2= 0.2^2+4*0=0.04

这样a+b=0.2 而either white or even 就是a+b-ab的概率，那不就求出来了？

麻烦大家看看我的思路错在哪里呢？

Shiry

继续问下prep2中的两道DS题，拜托NN们帮忙指点下啊

if y is an integer and y=/x/+x,is y=0?  （/x/是x的绝对值）
1. x<0
2. y<1

答案是D  我觉得是A

这题没有思路，始终觉得条件2没有办法得出结论

SET S consists of five consecutive integers, and set T consistsof even consecutive integers, Is the median of the number in set S equal to the median of the median of the numbers in set T?
1.the median of the number in set is 0
2. the sum of the numbers in set S is equal to the sum of the numbers in set T.

答案是C  我认为是B

思路是这样的，分别设S、T数列的中数为X、Y

那么S的总和是5X ；T的总和是5Y 因为前后的数字大小相差刚好都抵消了

这样不就得出median相等了吗 条件1有什么用呢

Shiry

大家帮忙看下 给点思路就好^_^

snowyin

SET S consists of five consecutive integers, and set T consistsof even consecutive integers, Is the median of the number in set S equal to the median of the median of the numbers in set T?
1.the median of the number in set is 0
2. the sum of the numbers in set S is equal to the sum of the numbers in set T.

答案是C  我认为是B

这个要仔细看题，其实条件2只是说明连个数列和相等，但是两个数列包括的数和有多少这个数是不同的，这样的话其实答案就不只一个了。加上条件2，就可以得到唯一的答案。

不知道对不对，各位nn进来看看

buda

又翻了下prep 发现DS 的原题如下：

each of the 25 balls in a certain box is either red, blue or white and has a number from 1 to 10 painted on it. If one ball is to be selected at random from the box, what is the probability that the ball selected will either be white or have an even number painted on it?
1. the probability that the ball will both be white and have an even number painted on it is 0
2. the probability that the ball will be white minus the probability that the ball will have an even number painted on it is 0.2

从条件2可以推出白球概率为0.7，偶数概率为0.5，either be white or have an even number 是不是就是说两个概率相加，结果1.2》1，说明两个有重的，但是条件一很显然说没有重合，所以两个都没戏。合起来也没戏。不知道我的想法对不对。

buda

if y is an integer and y=/x/+x,is y=0?  （/x/是x的绝对值）
1. x<0
2. y<1

答案是D  我觉得是A

这题没有思路，始终觉得条件2没有办法得出结论

条件2：Y<1又是整数，所以只能是0或者负数，但是y=/x/+x只能是0或者正数，所以Y只能等于0

buda

SET S consists of five consecutive integers, and set T consistsof even consecutive integers, Is the median of the number in set S equal to the median of the median of the numbers in set T?
1.the median of the number in set is 0
2. the sum of the numbers in set S is equal to the sum of the numbers in set T.

答案是C  我认为是B

思路是这样的，分别设S、T数列的中数为X、Y

那么S的总和是5X ；T的总和是5Y 因为前后的数字大小相差刚好都抵消了

这样不就得出median相等了吗 条件1有什么用呢

是不是看题错了？set T consistsof even consecutive integers, 明明是说偶数个连续整数，为什么要取T的总和是5Y ？