Then, since A, B are rational and commensurable in square only,
the rectangle A, B, that is, the square on C [VI.17], is medial. [X. 21]
Therefore C is medial. [X. 21]
And since, as A is to B, so is C to D, and A, B are commensurable in square only, therefore C, D are also commensurable in square only. [X. 11]
And C is medial;
therefore D is also medial. [X. 23, addition]