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PREP的DS，伤人呀~~求助~~
perp ds1 做了两百来个，对了答案，错误多也就罢了，答案也看不懂呀。。 也不知prep破解的答案是怎么来的，某些题怎么都觉得答案别扭。因为自己向来讨厌数学外加粗心大意，只看狗狗不放心，练练prep也不知有没有效果。。。下周三考试，最近还有练作文，数学需要把prep或GWD啥的练练么？
以下是刚才对答案的几个问题，求帮助：
159.
If n is a positive integer and r is the remainder when (n - 1)(n + 1) is divided by 24, what is the value of r ?
(1) n is not divisible by 2.
(2) n is not divisible by 3.
【答案】C
【思路】
=
(1) n = 3 , 5 , 7 ,9…… ,則r=8 , 1 ……,不充份
(2)n=1 , 2 , 4,….,r=2 ,3………,不充份
(1) + (2) 不能被2 , 3整除 ,2 ,3 為 24 的公因數 ,所以n排除為24因數後,r=0 （这里是怎么回事呀，不明白。。 ）
165.
Of the 1,400 college teachers surveyed, 42 percent said that they considered engaging in research an essential goal.How many of the college teachers surveyed were women?
(1) In the survey, 36 percent of the men and 50 percent of the women said that they considered engaging in research an essential goal.
(2) In the survey, 288 men said that they considered engaging in research an essential goal.
【答案】A
【思路】(1) total（1400) x
=588(參加人數） , 588 x women比例（)=參加女人的比例 , 充份
(2)原題問surveyed werer women , 包含 engage 和 not engage , so we only can get engage women , 不充份
（此题自认为答案错误，应选C：（1）36%M+50%W=40%*1400 （2）288=36%*M）
167
If x, y, and z are integers and xy + z is an odd integer, is x an even integer?
(1) xy + xz is an even integer.
(2) y + xz is an odd integer.
（分情况讨论着讨论着就晕乎了，不会处理此种多情况讨论的题，求思路点拨）
178.
If there are more than two numbers in a certain list, is each of the numbers in the list equal to 0 ?
(1)The product of any two numbers in the list is equal to 0.
(2)The sum of any two numbers in the list is equal to 0.
【答案】B
【思路】(1)任何數相乘只要數列有一個0,乘積為0,但不代表數列均為0,不充份
(2)任何數相加為0,其必為兩個0,充份
（我选的是D，感觉答案对于（1）的解释是错误的啊。。？） |
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