Hi I'm trying to show that for a commutative ring with characteristic p that

(a + b)^p = a^p + b^p

I have verified this with a few examples but can't seem to prove that the binomial coefficients C(p,r) = 0 (modp) where r /= 0 or p (i.e. the first or last term)

Can anyone help me out? Cheers =)