Yes, thanks, I knew it was out there, but thought it's an interesting result to share. I heard too that it has a generalization for higher dimensions, like cubes etc, which your citation mentions as well.

Here, I found Monsky's original paper/proof:

http://ieee.scripts.mit.edu/urgewiki/images/0/00/Monsky.pdf

where he shows that in fact the areas of the dissected triangles satisfy some polynomial relation. His proof is just about 2 pages.

Posted January 28th
by The Fly