zhangws

159.   If n is a positive integer and r is the remainder when (n - 1)(n + 1) is divided by 24, what is the value of r ?

(1) n is not divisible by 2.

(2) n is not divisible by 3.
------------------------------------------------------------------------------------------------------------------【答案】C

【思路】 =
       (1) n = 3 , 5 , 7 ,9…… ,則r=8 , 1 ……,不充份
     (2)n=1 , 2 , 4,….,r=2 ,3………,不充份

(1) + (2) 不能被2 , 3整除 ,2 ,3 為 24 的公因數 ,所以n排除為24因數後,r=0