I'm reading the book Mathematics: It's Content, Methods, and Meanings and I'm unsure as to how one of the variables in an example was derived. The question is about the volume of a paraboloid and here is the text and diagram. How is the the radius the sqr (z)? It claims it is quite obvious but I'm unable to understand. I think the volume of the bottom cylinder is the area of the bottom circle times delta z and the volume of the top cylinder is the area of top circle times delta z but that is just a guess and still leaves me the question of how the radius was derived from the height.