The proof given above is basically the one given on pp. 45-46 of Geometry Revisited,
by H.S.M. Coxeter and S.L. Greitzer, New Mathematical Library Vol. 19, Random House,
1967. The authors say they received this proof from a “Dr. Zoll of Newark State College.”
This proof is pretty common nowadays, probably because it is pretty basic in the tools it
requires; e.g. the Wikipedia article on the Butterfly Theorem gives a version of it.
right angles
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