We describe a new proof of the well-known Lyapunov's matrix inequality abou
t the location of the eigenvalues of a matrix in some region of the complex
plane. The proof makes use of standard facts from quadratic and semidefini
te programming. Links are established between the Lyapunov matrix, rank-one
linear matrix inequalities (LMIs), and the Lagrange multiplier arising in
duality theory.