TestMagic Forums - View Single Post - Is n-1 divisible by 3?

12rk34 · 2009 February 27th, 01:08 AM

(1) n*(n+1) is not divisible by 3.
This is possible if n=3k+1 where k is any integer (k=-3, -2, -1, 0, 1, 2...).
n*(n+1) = (3k+1)(3k+2)
= 9K^2 + 9k + 2 is not divisible by 3 since last term i.e. 2 is not divisible by 3.
Obviously n-1 =3k is divisible by 3.
Hence A.

2009 February 27th, 01:08 AM	#3 (permalink)
12rk34 TestMagic Guru Join Date: Jun 2008 Posts: 1,566	(1) n(n+1) is not divisible by 3. This is possible if n=3k+1 where k is any integer (k=-3, -2, -1, 0, 1, 2...). n(n+1) = (3k+1)(3k+2) = 9K^2 + 9k + 2 is not divisible by 3 since last term i.e. 2 is not divisible by 3. Obviously n-1 =3k is divisible by 3. Hence A.