好心人帮帮我吧 不会了 谢谢谢！！！！1

guo573874201

1.Juan bought some paperback books that cost $8 each and some hardcover books that cost $25 each.If Juan bought more than 10 paperback books, how many hardcover books did he buy?
(1)The total cost of the hardcover books that Juan bought was at least $150.
(2)The total cost of all the books that Juan bought was less than $260.答案选C

2.Is x greater than y ?
(1)x – y² > 0
(2)xy < 0答案选C  怎么都不会是C呀  我选的E   是我拿错了吗

3.A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetic order.What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?答案选B    什么意思呢 没有看懂啊  
A.4
B.5
C.6
D.7
E.8

may24

2.Is x greater than y ?
(1)x – y2 > 0  推出 x >y2 >0
(2)xy < 0   推出x,y互为正负
（1）+（2）推出 x >0 >y

houbinpeter22

1. Juan brought more than 10 paperbacks, which means Juan spent at least $88 on paperback. Either option 1or 2 alone can not solve this problem.
When you consider these two toptions at the same time, by using try and error, you will get she brought 6 hardbacks. 150/25=6

2.  (1) when x is 0.5, and y is 0.6; x is 5, and y is 1, these two combination are all true for (1), then, cannot solve the problem.
(2) alone can not solve the problem too.

Take both 1 & 2 into consideration:
From (2), we can know that the sign of x and y are opposite.  From (1) we can know that x is positive number. Thus, y must be negative. Which Solve the problem.

3. 这题意思就是在一个实验中，为了区分开参加者，实验者用字母标识每一个参加者。标识的方法是一个字母对应一个参加者，或2个不同的字母按照字母顺序标识参加者。 例如。 用a,b,c 三个字母就可以标识6个参加者。 依次是： a, b, c, ab, ac, bc
题目问当有12个参加者时，最少要几个字母

guo573874201

您好  那第三题有什么简便方法可以算呢？

houbinpeter22

也就是从第一个选项开始试的

4个字母（eg. A,b,c,d). 就可以组成 4+3+2 = 9种不同的分类标识

4： a, b, c, d.        3:  ab, ac, ad.      2: bc, bd

5个字母 道理一样， 可以组成 5+4+3+2=14 种. Correct