We shall prove by induction that Sn is a linearly independent set of A. For
n = 1 we have S1 = {1, x}, and let us consider a zero linear combination of
the two elements of S1, that is α1 + βx = 0 with α, β ∈ F. It follows that
D(α1+βx) = 0, and hence β · 1 ·D(x) = 0. From the assumption that D(x) is
not a zero divisor, it follows that β = 0, which in turn implies that α = 0. It
follows that S1 is a set of linearly independent vectors. Now let us note that
from the assumption that xD(x) = D(x)x, we have