TTGWD4-15

[fsab00037748]

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.

答案給C

請問這題怎麼做呢?

[Emmi]

(1) 2 is not a factor of n. -> n is not even
又 (2) 3 is not a factor of n. -> n = 1,5,7,11,13,17,.....

則, n=1 => (n-1)(n+1)=0
n=5 => (n-1)(n+1)=4*6=24
n=7 => (n-1)(n+1)=6*8=48
.....
類推,皆可被24整除, r=0

[fsab00037748]

嗯嗯 謝謝你,

不過如果要這樣做的話, 幾乎是得大概有個"感覺" 兩個條件相加可以解出後
然後再去類推答案試試....偏偏...我就常沒這種感覺@@!

不知道這類型的題目有沒有除了如此類推外的做法呢?