If there be as many numbers as we please in continued proportion, and the first measure the last, it will measure the second also.
Proof
If a1∣an, reduce a1,…,an to lowest terms (VIII.3);
since the lowest extremes are coprime but a1 divides an, the
ratio a1:an must be 1:1 in lowest terms, forcing a1∣a2 via VII.20.