The last question may not be clearly written. Let me tell you what I think. With the increase of the cone z, z= root sign (x 2+y 2), that is, the radius of the arc projected by the cone on the xy plane is increasing, and then the idea of infinitesimal method. When z increases from r to r+Δ r, the increased part of the graph is the graph of the height of r and the bottom surface of arc AC multiplied by Δ r.