# Homework Help: Proof involving divisibilty

1. May 2, 2010

### GP35

1. The problem statement, all variables and given/known data

Show n4 - 1 is divisible by 5 when n is not divisible by 5.

2. Relevant equations

3. The attempt at a solution

My thought here is proof by cases, one case being where n is divisible by 5 and the other case saying n is not divisible by 5. I am just not totally sure how to implement this strategy.

2. May 2, 2010

### Martin Rattigan

If you split your second case a bit more that will do fine.

3. May 3, 2010

### HallsofIvy

If n is not divisible by 5, then it is of the form n= 5k+ 1, n= 5k+ 2, n= 5k+ 3, or n= 5k+ 4.

Or you can consider n= 5k- 2, n= 5k- 1, n= 5k+ 1, and n= 5k+ 2 if that makes the calculations simpler.

4. May 4, 2010

### icystrike

If we would to spent a little more time to think through , we will realise that the determining factor will then lies in $$5k \pm m$$ such the $$m^{4}\equiv 1 (mod5)$$ with application with binomial theorem.