The composite beam AB consists of three segments. The segments AC and DB are both rigid and of length a, while the segment CD is of flexural rigidity, EI and length (L − 2a). The beam is simply supported at A, B and compressed by a force P. Find the equation determining the critical value of P for instability and show that your result reduces to P0=π2E/L2 when a = 0. Solve the equation for the case where a = L/4.