dervabk(t,x);

function xd = dervabk(t,x);
% this function returns the derivatives of the unit feedback system
% described by the equation:
% x'      =  A*x + B*u
% where u = -K*x
% the calling function must contain the definition of the
% matrixes A B K and the statement
% global A B K;

% Delete next line, if you are using a MATLAB version prior to 4.0.
global A B K;
u  = -K*x;
xd = A*x + B*u;