求助！看了解释都不明白的PR Hard Math中两道

chailin45

1。Let M be the maximum value and N be the minimum value of the expression (X^2+Y)/ Y. If a<=X<=b and c<=Y<=d then what is the value of M - N?
 
(1)  c = 3
(2)  a = -2, b = 5 and d = 7

答案是B(2单独成立)，解释说In order to find the minimum, if x could equal 0 (implied by its range covering both negative and positive number), then we know the minimum of the expression too: no matter
what y equals, the result will be 1.

我的问题，要是Y< 0 ，式子值不就小于1了吗？怎么会和Y的最小值d 没有关系呢？？ 不明白。。。
 

2。

Set B has three positive integers with a median of 9.  If the largest possible range of the three numbers is 19, given a certain mean, what is that mean?

(A)  22

(B)  10

(C)  9.6

(D)  9

(E)  It cannot be determined from the information given

答案是B
为什么不是E，有很多可能啊？！！

附件中是带详细讲解的PR Hard Math，解释在每篇题的下面，CD好像找不到了，刚发了份在下载区，顺带也放在这~