ViewTubes

Here is a dynamic version of the ViewTubes problem
a) What is the visible diameter when the tube is against the wall? How far away is the eye at this point? b) Write a function for the visible diameter as a function of eye distance from the wall. c) Write a function for the visible diameter as a function of tube distance from the wall. d) How are your answers in (b) and (c) related? Why?