[求助]求问prep几题，十分感谢~

小女水平有限，求问几道prep数学

望好心人指点思路，不胜感激~~

1.

A certain stock exchange designates each stock with a one-, two-, or three-letter code, where each letter is selected from the

26 letters of the alphabet.  If the letters may be repeated and if the same letters used in a different order constitute a different code, how many different stocks is it possible to uniquely designate with these codes?

(A)  2,951

(B)  8,125

(C)  15,600

(D)  16,302

(E)  18,278

2.

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)

3.If xy=1,what is the value of ［2(x+y)^2 ］/［2(x-y)^2］

(A) 2

(B) 4

(C)8

(D) 16

(E) 32

第一题 26+26*26+26*26*26

第二题 26^4+26^5

最后一题你确定你的符号表示的没问题？？

谢谢解答！！！

最后一题我做两次prep1都出现了，我一直以为自己眼睛出问题了~~~

可是题目就是这么个东西~~~

26^4表示是26的4次方

好像是这样

记得不是很清楚了