function [t,x]=feuler(f,x0,ti,tf,h)
  t=[ti:h:tf];
  x=zeros(length(x0),length(t));
  x(:,1)=x0;
  for k=1:length(t)-1
    x(:,k+1)=x(:,k)+h*f(x(:,k),t(k));
  end
end