Euclid's Elements - Book 8, Proposition 8

Propositions: If A:B = C:D, then the length of the proportional series A, S1, S2 ... Sn, B, will be equal to length of the proportional series C, T1, T2 ... Tn, D PDFs: https://github.com/sandy-bultena/Euclid/tree/master/Propositions/PDFs