Because Sum_{i=0}^{k} a_i = a_0 + Sum_{i=1}^{k} a_i. When k = 0, the left side equals a_0, while the right side equals a_0 plus the empty sum. Similarly for p

Because Sum_{i=0}^{k} a_i = a_0 + Sum_{i=1}^{k} a_i. When k = 0, the left side equals a_0, while the right side equals a_0 plus the empty sum. Similarly for products.