What is the X in X-mas?
The theorem of geometry, named after Pythagoras, of Samos, Ionia, stating that, for a right-angled triangle, the square of the length of the hypotenuse is equal to the sum of the squares of the lengths of the other two sides. I.e. if the longest side has length A and the other sides have lengths B and C (in any units),
A^2 = B^2 + C^2