# Where do I start?

1. Apr 7, 2005

### recon

Can someone point me in the right direction of solving the following problem:

Prove that for any postive integer n, the value of the expression $$3^{2n+2} - 8n -9$$ is divisible by 64.

Last edited: Apr 7, 2005
2. Apr 7, 2005

### honestrosewater

I find when you're asked to prove that every member of some subset of the natural numbers has some property, induction usually works.

3. Apr 7, 2005

### matt grime

9^{n+1}-8n-9

or equivalently

9^n-8n-1

what is the binomial expansion of 9^n when considering 9=8+1?

4. Apr 7, 2005

### honestrosewater

Maybe I'm having a blonde moment, but doesn't $a^{n + 1} - a = a(a^{n} - 1)$, making $9^{n + 1} - 9 - 8n = 9(9^{n} - 1) - 8n$?

5. Apr 7, 2005

### matt grime

ah, perhaps i ought to have been clearer: i wasn't say the epxressions are equal, but that if you prove one is divisible by 64 for all n, the other will be divislbe by 64 for all n (give or take a case when n=0). I let m=n+1 in the first, then relabelled n=m.

6. Apr 9, 2005

### recon

I get it now. Thank you.