BibTeX: 
@article{Ptolemys_TheoremAFP,
author = {Lukas Bulwahn},
title = {Ptolemy's Theorem},
journal = {Archive of Formal Proofs},
month = aug,
year = 2016,
note = {\url{https://isaafp.org/entries/Ptolemys_Theorem.html},
Formal proof development},
ISSN = {2150914x},
}
