I'm doing some research on space elevators and have found a site (http://www.zadar.net/space-elevator/#transverse) which gives some insight into the maths behind the elevator cable. If you click on the link and scroll down, up, whichever direction to equation (2) and a picture of a blue trapezium the guy has found the difference in the volume of the tapering cable with what looks like volume of revolution, but I cant seem to figure out what he's done. He says that it's 'easy to show...'. Am I missing something? I could use a little help. thanks.