The three values of the sides of a right angled triangle are also called a Pythagorean Triple.

A Pythagorean triple is a set of three whole numbers , such that one number squared added to another number squared equals the third number squared. Euclid could prove that there are an infinite number of such Pythagorean triples.

Euclid's proof begins with the observation that the difference between successive square numbers is always an odd number.

4 - 1 = 3, 9 - 4 = 5, 16 - 9 = 7, 25 - 16 = 9, 36 - 25 = 11, 49 - 36 = 13 etc.