cat模考错题求解答

Owen’dream

[size=12.0012px]x is the sum of y consecutive integers. w is the sum of z consecutive integers. If y = 2z, and y and z are both positive integers, then each of the following could be true EXCEPT

此题不能假设z=1是因为w  is the sum of z consecutive integers？ 是这么理解吧？

[size=12.0012px]

	a	x = w
	b	x > w
	c	x/y is an integer
	d	w/z is an integer
	e	x/z is an integer

humingyi

y = 2z 说明 y是偶数，
偶数个连续整数的平均均属必为 n.5的形式。 e.g. 1 和 2 的平均 1.5.
x/y就是这y个数的平均数，必不为整数

xxxxxxiomo

Owen’dream 发表于 2016-12-22 04:09
明白！感激！但是假如ds问题是这种形式： x>3？那么条件x=3也是sufficient的么 ...

对 只要保证答案的唯一性

Owen’dream

xxxxxxiomo 发表于 2016-12-22 03:51
这题答案是b吧？
(1)是明显不行的 分别带入2和3，发现有两种情况。
(2)是可以的 因为 x是odd并且大于1，x ...

明白！感激！但是假如ds问题是这种形式： x>3？那么条件x=3也是sufficient的么

xxxxxxiomo

Owen’dream 发表于 2016-12-22 03:33
另外这道ds题困扰了我好久sufficient的定义到底是什么

If x is a positive integer greater than 1, is x! ...

这题答案是b吧？
(1)是明显不行的 分别带入2和3，发现有两种情况。
(2)是可以的 因为 x是odd并且大于1，x!中一定包含even，所以x!为even，x＋1也为even 所以整个式子都是even。even cannot be prime. thus, the answer is b.

sufficiency 就指得是 通过运用 两个条件 来解答问题是否有唯一的解。这就是为什么选b的原因。因为b只存在唯一一个解，which is x! + x + 1 CANNOT be a prime number.我也不知道我表述的清不清楚 >_<

Owen’dream

Six mobsters have arrived at the theater for the premiere of the film “Goodbuddies.”  One of the mobsters, Frankie, is an informer, and he's afraid that another member of his crew, Joey, is on to him. Frankie, wanting to keep Joey in his sights, insists upon standing behind Joey in line at the concession stand, though not necessarily right behind him.  How many ways can the six arrange themselves in line such that Frankie’s requirement is satisfied?
此题怎么翻译。。。f和j的位置到底闹哪样

Owen’dream

另外这道ds题困扰了我好久sufficient的定义到底是什么

If x is a positive integer greater than 1, is x! + x + 1 a prime number?

(1) x < 10

(2) x is odd