For let C by multiplying itself make E, and let D by multiplying itself make G; further, let C by multiplying D make F, and let C, D by multiplying F make H, K respectively. Now it is manifest that E, F, G and A, H, K, B are continuously proportional in the ratio of C to D. [VIII. 11, 12] And, since A, H, K, B are continuously proportional, and A measures B, therefore it also measures H. [VIII. 7] And, as A is to H, so is C to D; therefore C also measures D. [VII. Def. 20] Next, let C measure D; I say that A will also measure B. For, with the same construction, we can prove in a similar manner that A, H, K, B are continuously proportional in the ratio of C to D. And, since C measures D, and, as C is to D, so is A to H, therefore A also measures H, [VII. Def. 20] so that A measures B also. Q. E. D.
Euclid. Euclid's Elements. Sir Thomas Little Heath. New York. Dover. 1956.
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