Prove that 5^(2n - 1) is divisible by 8

Your teacher must have made a typo somewhere. The expression 5^(2n-1) is not divisible by 8.

The expression 5^(2n-1) is composed of 2n-1 copies of "5" multiplied together. The key here is that 5 is the only unique prime factor of this number.

On the other hand, 8 = 2*2*2 = 2^3 showing that 2 is the only prime factor for this number. At this point, we can see that there's no way that 5^(2n-1) is divisible by 8. It all comes down to the clash of the primes 5 and 2 not matching up, and having nothing in common.

Let's consider a few concrete examples:

I'll let you try other natural numbers for n, and you'll find that isn't an integer.