When two medial (areas which are) incommensurable with one another are added together, the remaining two irrational (straight lines) arise (as the square root of the total area) - either a second bimedial, or the square root of (the sum of) two medial (areas). * For let the two medial (areas) $AB$ and $CD$, (which are) incommensurable with one another, have been added together. * I say that the square root of area $AD$ is either a second bimedial, or the square root of (the sum of) two medial (areas).
(not yet contributed)
Proofs: 1