If a magnitude has a given ratio to a part of it, it shall also have a given raito to the remaining part of it.*
Let the magnitude AB have a given ratio to AC a part of it; it has also a given ratio to the remainder BC.
Because the ratio of AB to AC is given, a ratio may be found (2. def.) which is the same to it: let this be the ratio of DE a given magnitude to the given magnitude DF. And because DE, DF are given, A C B
the remainder FE is (4. dat.) given: and 1
because AB is to AC, as DE to DF, by D F E
conversion (E. 5.) AB is to BC, as DE 1
to EF. Therefore the ratio of AB to BC is given, because the ratio of the given magnitudes DE, EF, which is the same with it, has been found.