function z = df(x,y);
%
% for second-order Taylor, I want to compute
%
%          f_y(x,y(x)) f(x,y(x)) + f_x(x,y(x))
%
% since f(x,y) = cos(x)*y, f_y = cos(x) and f_x = -sin(x)*y

f_x = -sin(x)*y;
f_y = cos(x);
z = f_y*(cos(x)*y) + f_x;
