Example 4 (p. 733)
> F := -c * [x,y,z] / ( x^2+y^2+z^2 )^(3/2);
> f := c * (x^2+y^2+z^2)^(-1/2);
> gradf := grad( f, [x,y,z] );
>
> evalm( F-gradf );
![F := -c*[x, y, z]/(x^2+y^2+z^2)^(3/2)](images/sec17-17.gif)
![VECTOR([0, 0, 0])](images/sec17-110.gif){kind=small}