function outv=trap1step(a,b,n,oldval)
%outv=trap1step(a,b,n,oldval)
%
%This routine calculates one step of the Trapeziodal Method to
%approximate the definite integral of a function, myf, over the
%interval [a,b], using n subintervals.
%
%It assumes that a previous approximation using n/2 subintervals
%has been performed and the result is in the parameter oldval.
%
%Inputs:
%   a,b - the endpoints of the interval of integration.
%   n   - the number of subintervals used in the approximation.
%   oldval - the Trapeziodal Approximation using n/2 subintervals.
%   myf - not listed, a user defined function (M-file called myf.m)
%
%Outputs:
%   outv - the approximation.
global count;

h=(b-a)/n;                      %h is the length of the subinterval.
ii=[1:2:n-1];                   %ii is an index of the "new" nodes.
xx=a+h*ii;                      %xx is the values of the "new" nodes.
outv=oldval/2+h*sum(myf(xx));
%count=count+length(ii);