On 7/20/2022 8:33 AM,
henh...@gmail.com wrote:
> Take an odd integer, square it, and subtract 1 ( One )... then
> the result is always a multiple of ____________ !
[spoiler space]
For any odd integer x:
x = 2y + 1 (for some integer y)
x^2 = 4y^2 + 4y + 1
x^2 - 1 = 4y^2 + 4y
= 4 * y * (y+1)
Obviously it's divisible by 4, but also either y or y+1 is divisible by
2 (if one is odd then the other is even), so x^2 - 1 is actually always
divisible by 8.
Sanity check:
1^1 - 1 = 0 = 8 * 0
3^1 - 1 = 8 = 8 * 1
5^1 - 1 = 24 = 8 * 3
7^1 - 1 = 48 = 8 * 6
9^1 - 1 = 80 = 8 * 10
and we can recognize the rightmost terms as
y * (y+1) / 2
for y = 0, 1, 2, 3, 4, etc.