请教两题数学题，小女子不胜感激～

小豌豆

第一题是做prep上遇到的：The function f is defined for all positive integers n by the following  rule: F(n) is the number of positive integers each of which is less than n and has no positive factor in common with n other than 1. If P is any prime number then f(P)=?
A, p-1    B, p-2     C, (p+2)/2     D, (p-1)/2   E,2
(我完全没有思路啊～）


第二题：一道关于排列组合的题
A certain company has 18 equally qualified applicants for 4 open positions. How many different groups of 4 applicants can be chosen by the company to fill the positions if the order of selection does not matter?
我理解的和题目也是不一样啊！答案是 3060，求过程～


ps, CDers 都加油，都美梦成真

小豌豆

醍醐灌顶，呵呵，超级感谢拉，讲解的很详细～第二题是被我想复杂了～

小豌豆

谢谢拉，就是a拉