[求助]TT GWD-4-15

Q15:

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)     2 is not a factor of n.

(2)     3 is not a factor of n.

答案是C我选E，请指教

r=0

过程1 列几个书看一下就行，n=1,5,7,11,13....时，发现都有r=0

方法2 n是奇数，则(n-1)(n+1)中必有4或者8的倍数，包括0；3不是n的因数，则(n-1)(n+1)中必有6或者3的倍数。所以，(n-1)(n+1)必是24的倍数，r=0.