The surface of a lake is represented by a region D in the xy plane so that the depth (in meters) below the point corresponding to (x, y) is f (x, y) = 300-2x^2-3y^2. It is in the water at point (4,9) - IN which direction must it swim so that the depth of the water under it decreases more quickly? In which direction will the depth remain constant? D is the direction.