Euclid's Elements, Book 1, Proposition 47:
"In right-angled triangles the square on the side opposite the right angle equals the sum of the squares on the sides containing the right angle". Also known as "Pythagorean theorem".
http://www.pbase.com/adalberto_tiburzi/image/24335717 Grazie Luca, idea geniale ;-)
Adal