http://aleph0.clarku.edu/~djoyce/elements/bookV/propV23.html
GEOMETRY IS MUCH BROADER THAN MERE MATHEMATICS
"Parts have the same ratios as their equimultiples," means that one slice of pound cake consists of like ratios of its constituent ingredients in any number of other slices or in the whole itself .
This is derived from EUCLID'S Elements, book 5, proposition 23