yes p is the smallest integer such that 1+...+1 = 0 (p times)
0< r < p
*therefore* you deduce that r is not zero, because it is smaller than p
so becaues ra_k = 0, this *implies* that a_k = 0
*if* for some reason you know that a_k is not zero, *then* you get a contradiction (since you're working in a field there are no zero divisors)