The diagonal divides its square into equal halves
A proportional argument on a lettered rectangle a, b, c, refuting an opponent's supposition
A faint page carrying a single geometric argument built on a lettered rectangle (with the small triangle-in-rectangle diagrams at lower right). Leonardo sets a b worth 4 and c worth 2, and shows that giving b to c makes b c equal to a by itself. He then refutes an imagined 'opponent' who supposes a b to be 5 and c to be 1, since that would make the straight diameter fail to divide the square into equal parts, contrary to the third proposition. The sheet bears further worn, untranscribed writing beyond this block.
On this page
The diameter must divide its square equally, refuting the opponent
With a b worth 4 and c worth 2, giving b to c makes b c equal to a alone. The opponent's claim that a b is 5 and c is 1 fails, for then the diameter would not divide the square into equal parts (a being 4 and b c being 2), against the third proposition proving the straight diameter divides its square equally.