Euclid's Elements, Book X, Proposition 22 (X.22)

Euclid's Elements, Book X, Proposition 22 "The square on a medial straight line, if applied to a rational straight line, produces as breadth a straight line rational and incommensurable in length with that to which it is applied." Lemma "If there be two straight lines, then, as the first is to the second, so is the square on the first to the rectangle contained by the two straight lines."