PREP-DS2-12,三个圈的集合题,请教!

12.       781-!-item-!-187;#058&000341

Sets A, B, and C have some elements in common.  If 16 elements are in both A and B, 17 elements are in both A and C, and 18 elements are in both B and C, how many elements do all three of the sets A, B, and C have in common?

(1) Of the 16 elements that are in both A and B, 9 elements are also in C.

(2) A has 25 elements, B has 30 elements, and C has 35 elements

谢谢

刚才做题,又发现一个整数题搞不明白,也一并来请教,很感谢回复!

IS 1/P >R/(R^2+2) ?

1)P=R

2)R>0

我选A,答案是C

我的思路:根据题目,得出r^2+2>pr,条件1说P,R相等,则题目肯定是大于关系呀.不需要再加上B的R是正数的条件呀.请指出我又是昏在哪儿了?谢谢

第一题A可以推出来三者交集为9吧，B推不出，具体怎么推我也比较糊涂 ：（；

第二题你可能没考虑不等式两边同时乘负数的话不等号是要变方向的，所以要加上R〉0才能成立。

谢谢SANDY,第二题,我怎么看不出来哪里需要两边同乘负数呢? 第一题我就是跟你糊涂的一样,呵呵.

希望还有同学能知道的.