Now we have the surface sag, the next step is to compute the direction cosines of the normal vector. The normal vector at a point (x₀,y₀) on a surface z = f(x,y) is given by

where f_x = df/dx and f_y = df/dy are partial derivatives. So, the direction cosines will be as follows:

In Maple, f_x is given by:

The same actions are used to compute F_y.