A first bimedial straight line is divded at one point only.

Let AB be a first bimedial straight line divded at C, so that AC, CB are medial straight lines commensurable in square only and containing a rational rectangle; I say that AB is not so divded at another point.

For, if possible, let it be divded at D also, so that AD, DB are also medial straight lines commensurable in square only and containing a rational rectangle.

Since, then, that by which twice the rectangle AD, DB differs from twice the rectangle AC, CB is that by which the squares on AC, CB differ from the squares on AD, DB, while twice the rectangle AD, DB differs from twice the rectangle AC, CB by a rational area—for both are rational— therefore the squares on AC, CB also differ from the squares on AD, DB by a rational area, though they are medial: which is absurd. [x. 26 ]

Therefore a first bimedial straight line is not divded into its terms at different points; therefore it is so divded at one point only.