Then, since AB, BC are unequal straight lines, and the square on AB is greater than the square on BC by the square on a straight line incommensurable with AB,
while there has been applied to AB a parallelogram equal to the fourth part of the square on BC,
that is, to the square on half of it,
and deficient by a square figure, making the rectangle AE, EB,
therefore AE is incommensurable with EB. [X. 18]
And, as AE is to EB, so is the rectangle BA, AE to the rectangle AB, BE,
while the rectangle BA, AE is equal to the square on AF, and the rectangle AB, BE to the square on BF;
therefore the square on AF is incommensurable with the square on FB;
therefore AF, FB are incommensurable in square.
And, since AB is rational, therefore the square on AB is also rational;
so that the sum of the squares on AF, FB is also rational. [I. 47]
And since, again, the rectangle AE, EB is equal to the square on EF,
and, by hypothesis, the rectangle AE, EB is also equal to the square on BD, therefore FE is equal to BD;
therefore BC is double of FE, so that the rectangle AB, BC is also commensurable with the rectangle AB, EF.
But the rectangle AB, BC is medial; [X. 21]
therefore the rectangle AB, EF is also medial. [X. 23, Por.]
But the rectangle AB, EF is equal to the rectangle AF, FB; [Lemma]
therefore the rectangle AF, FB is also medial.
But it was also proved that the sum of the squares on these straight lines is rational.
Complete text
Book I