We know that eat and te-at are fundamental solutions of the fol- lowing equation: d²y dy + a²y=0. (1) dx² + 2a dx Suppose that we only know one solution e-at of (1). Assume (e-at, y₁ (t)) is a set of fundamental solutions of (1). By Abel's theorem, we know the Wronskian of (1) is given by W(e-at, y₁) = cexp{-f2adt}, use the Wronskian to obtain a first order differential equation of y₁ and solve it to find the fundamental set of solutions of (1).