I want to read more about the amazing result that, when given a closed, convex curve in the plane that can be traversed internally by a chord of length $p$+$q$, and on that chord lying at p, a point, the path of which traces another curve inside the first, the area between these curves is $\pi$$p$$q$.
I have forgotten the name of this result. Haddart? I can't seem to find it. If someone knowns what I mean, please reveal.