then
a + c + e : b + d + f = a : b.
Prop. 13. If
a : b = c : d and c : d > e : f,
then
a : b > e : f.
Prop. 14. If
a : b = c : d, and a > c, then b > d.
Prop. 15. Magnitudes have the same ratio to one another that their equimultiples have—
ma : mb = a : b.