General

Anyone feel like practicing Calc?

A doughnut has been partially eaten by a meticulous person so that the portion remaining is given by rotating the half-circular region shown above about the y axis. What fraction of the original doughnut remains, assuming that the original doughnut was the volume of revolution of the full circle? Give your answer in exact form in terms of r and R.
[url=http://tinypic.com/view.php?pic=pb0nk&s=6]Graph[/url]
Please help.
Note: 2(pi^2)(Rr^2) is not the answer.