For AB is either greater or less than CD.
First, let it be greater;
let a rational straight line EF be set out, let there be applied to EF the rectangle EG equal to AB,
producing EH as breadth, and let HI, equal to DC, be applied to EF, producing HK as breadth.
Then, since AB is rational and is equal to EG, therefore EG is also rational.
And it has been applied to EF, producing EH as breadth;
therefore EH is rational and commensurable in length with EF. [X. 20]
Again, since CD is medial and is equal to HI, therefore HI is also medial.
And it is applied to the rational straight line EF, producing HK as breadth;
therefore HK is rational and incommensurable in length with EF [X. 22]
And, since CD is medial, while AB is rational, therefore AB is incommensurable with CD,
so that EG is also incommensurable with HI.
But, as EG is to HI, so is EH to HK; [VI. 1]
therefore EH is also incommensurable in length with HK. [X. 11]
And both are rational;
therefore EH, HK are rational straight lines commensurable in square only;
therefore EK is a binomial straight line, divided at H. [X. 36]
And, since AB is greater than CD, while AB is equal to EG and CD to HI, therefore EG is also greater than HI;
therefore EH is also greater than HK.
The square, then, on EH is greater than the square on HK either by the square on a straight line commensurable in length with EH or by the square on a straight line incommensurable with it.
First, let the square on it be greater by the square on a straight line commensurable with itself.
Now the greater straight line HE is commensurable in length with the rational straight line EF set out;
therefore EK is a first binomial. [X. Deff. II. 1]
But EF is rational;
and, if an area be contained by a rational straight line and the first binomial, the side of the square equal to the area is binomial. [X. 54]
Therefore the “side” of EI is binomial;
so that the “side” of AD is also binomial.
Next, let the square on EH be greater than the square on HK by the square on a straight line incommensurable with EH.
Now the greater straight line EH is commensurable in length with the rational straight line EF set out;
therefore EK is a fourth binomial. [X. Deff. II. 4]
But EF is rational;
and, if an area be contained by a rational straight line and the fourth binomial, the “side” of the area is the irrational straight line called major. [X. 57]
Therefore the “side” of the area EI is major;
so that the “side” of the area AD is also major.
Complete text
Book I