If a first magnitude have to a second the same ratio as a third to a fourth, and the first be greater than the third, the second will also be greater than the fourth; and if equal, equal; and if less, less.
Proof
By V.8 and V.13: if a>c, then a:b>c:b. Combined with a:b=c:d (the hypothesis), V.13 gives c:d>c:b, whence b>d by V.10.