Proving a theorem

The theorem Andrei Negut ’08 proved for Edward H. Friend ’50 about which you reported (Notebook, April 2) is well known. A proof can be found in College Geometry by Nathan Altshiller-Court, Johnson Publishing Co., 1925. Somewhat different proofs can be found in Problem-Solving Strategies by Arthur Engel, Springer, 2000. Engel (and others, e.g., H.S.M. Coxeter) attributes the problem and its solution to Napoleon Bonaparte.