求教三题！

lilyxiang910

1.这个题我图怎么都粘不上来，就是一个半圆

The figure above shows the dimensions of a semicircular cross section of a one-way tunnel.  The single traffic lane is 12 feet wide and is equidistant from the sides of the tunnel.  If vehicles must clear the top of the tunnel by at least ½ foot when they are inside the traffic lane, what should be the limit on the height of vehicles that are allowed to use the tunnel?

 

A.     5½ ft

B.     7½ ft

C.     8 ½ ft

D.     9½ ft

E.      10 ft

Answer:  B

2. 下面这个题我算出来是6：5，没有选项，到底该怎么解呢？

If a code word is defined to be a sequence of different letters chosen from the 10 letters A, B, C,      D, E, F, G, H, I, and J, what is the ratio of the number of 5-letter code words to the number of    4-letter code words?

 

A.      5 to 4

B.      3 to 2

C.      2 to 1

D.     5 to 1

E.      6 to 1

Answer: 
                            E

3.
                                    If n and k are positive integers, is n divisible by 6?

(1)     n = k(k + 1)(k - 1)

(2)     k – 1 is a multiple of 3.

                  

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

D. EACH statement ALONE is sufficient.

E. Statements (1) and (2) TOGETHER are NOT sufficient.

这道题怎么解我都觉得应该选B，为什么答案是A呢？

希望帮我解答一下，谢谢麻烦各位了！

wentaow2008

  第一题小弟没看懂，第二题pay attention to the word "different letters", so 第一种情况就是10*9*8*7*6, 第二种就是10*9*8*7，也就是6:1了。第三题的话，楼主对(1)条件不妨带几组数据,k可以从1开始，证明成立；而（2）条件，k等于几和n有什么联系呢？所以不成立啊。