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Prep 19, 41

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楼主
发表于 2008-2-20 04:07:00 | 只看该作者

Prep 19, 41

请解释Prep 19, 41.

Thanks!!!

19.        2463-!-item-!-187;#058&002761

For a finite sequence of nonzero numbers, the number of variations in sign is defined as the number of pairs of consecutive terms of the sequence for which the product of the two consecutive terms is negative.  What is the number of variations in sign for the sequence 1, -3, 2, 5, -4, -6 ?

 

(A) One

(B) Two

(C) Three

(D) Four

(E) Five

the answer is C

41.        5676-!-item-!-187;#058&004686

All of the stocks on the over-the-counter market are designated by either a 4-letter or a 5-letter code that is created by using the 26 letters of the alphabet.  Which of the following gives the maximum number of different stocks that can be designated with these codes?

 

(A) 2(26^5)

(B) 26(26^4)

(C) 27(26^4)

(D) 26(26^5)

(E) 27(26^5)

the answer is C

沙发
发表于 2008-2-20 20:35:00 | 只看该作者

19. 有几组相邻乘出来时负的,

1 * -3

-3 * 2

5 * -4

ans.3

20. 4 letter or 5 letter

4 letter   26^4

the fifth: 26 letters + null = 27

ans:  (26^4) * 27

板凳
 楼主| 发表于 2008-2-21 01:58:00 | 只看该作者

Hi Ciscogeek,

Thanks for answering my question.

Can you explan the last question more. I think that 5 letter is 26^5.

why the fifth: "26 letters + null = 27" we need to add null here.

Thanks in advance!

地板
发表于 2008-2-21 10:52:00 | 只看该作者

the original formula is like:

26^4+26^5=26^4*(1+26)=27*26^4

null stands for the situation in which only 4 letters were used to designat the stock

5#
 楼主| 发表于 2008-2-21 11:56:00 | 只看该作者

Thanks Barett!

I got it.

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