function df=MidPoint(func,x0,h)
if nargin == 2 
    h = 0.1;
else if (nargin == 3 && h == 0.0)
        disp('h不能為0！');
        return;
    end
end

y1 = subs(sym(func), findsym(sym(func)),x0+h);
y2 = subs(sym(func), findsym(sym(func)),x0-h);
df = (y1-y2)/(2*h);