Solution: First, we find the scalar function that gives the distance between the two points. Then, we apply the gradient to this function. Because the coordinates (x,y,z) or (x0,y0,z0) may be taken as the variables, the gradient with respect to each set of variables is calculated. In applications, one point may be fixed while the other varies, so there may not be a need to calculate the gradient with respect to both sets of variables.