Not surprisingly, over the years this beautiful result has attracted the attentions
of many mathematicians, and there are several proofs known of it [1]
Fagnano himself apparently used differential calculus to prove it, though, by modern standards, it seems to me that this is far from being a routine exercise. Perhaps the
most appealing proofs of the theorem are those based on the Reflection Principle,
and two of these, in particular, due independently to L. Fej´er and H. A. Schwarz,
have made their appearance in several books aimed at general audiences [2],[3],[4] .A proof based on vector calculus appeared recently [5]. The purpose of
this note is to offer one based on trigonometry