function  [int,err]=update_trap(f,a,b,n,oldint)
%
%  function  [int,err]=update_trap(f,a,b,n,oldint)
%    Doubles number of points for Trapezoid Rule.
%    Assumes n is divisible by 2.
%
       h=(b-a)/n;
       int=0.5*oldint; sum=0;
       for j=1:2:n-1
        xx=a+j*h;
        sum=sum+ feval(f,xx);
       end;
       int=int+h*sum;
       err=(int-oldint)/3;