Let A be an operator with domain D(A) = {v C() (0, b) : v(0) = v(b) = 0} (Dirichlet type boundary conditions). If ker(A) = {0}, then calculate the Green function G(x, s) and determine the solution of the problem Au = f, where f = 2 cos x, under the boundary conditions u(0) = u(20) = 0. Graph the solution obtained and compare with the numerical solution calculated through the method of finite differences. Please write all steps