The paper considers the motion of a system whose state can be represented by the equations \[ \dot x=A(t)x+B(t)u \] Here it is shown how the approximate method for determining the fundamental matrix can be used to find the matrix $E(t_k)$ and $F(t_k)$ in discrete models \[ x(t_{k+1})=E(t_k)x(t_k)+F(t_k)u(t_k). \]