% uses the midpoint rule to estimate f''(x) 
%
% x is the point the derivative is being approximated at, h is the 
% interval distance the approximation is over.  Requires suboroutine
% f.m
%
% function y = dd_mid(x,h)

function y = f(x,h)

y = (f(x+h)-2*f(x)+f(x-h))/(h^2);