For those that have studied geometry, such as my students taking their final exam today, the Pythagorean Theorem is one of the most memorable formulas:
A2 + B2 = C2
The great Geometer Euclid, of course, derived this Theorem in Proposition 47 of Book I of his Elements.
There is no day like today to derive the Pythagorean Theorem, right?
Euclid the Geometer in the School of Athens by Raphael.