we obtain the factorization s 2 -(a + d)s + ad -bc = (s -λ 1 )(s -λ 2 ). Partial fractions expansion (using TI-Nspire expand() if one chooses) and the identity a + d = λ 1 + λ 2 allows one to recover the Putzer formula for e At found in and finally the exact solution to (4) using .