%  partials of a saddle surface

   xgrid = -2:0.2:2;
   ygrid = -2:0.2:2;
 
    [x,y] = meshgrid(xgrid, ygrid);
    Z = x.*y;
     surf(x,y,Z);
     
     
        colormap copper;
        hold on
  for x0 = -2:0.5:2,    
     %  x0 = 1;
     y0 = 1;
     z0 = x0*y0;
     ddx = y0;
     ddy = x0;
     
         t = -2:2;
      plot3( x0 + t*0, y0 + t, z0 + t*ddy, 'LineWidth',2)     % parametric equation of the tangent line when x = const   
  end    
     
  
  hold off