If four numbers be in continued proportion, and the first be cube, the fourth will also be cube.
Let A, B, C, D be four numbers in continued proportion, and let A be cube; I say that D is also cube.
For, since between A, D there are two mean proportional numbers B, C, therefore A, D are similar solid numbers. [VIII. 21]
But A is cube; therefore D is also cube. Q. E. D.
Euclid. Euclid's Elements. Sir Thomas Little Heath. New York. Dover. 1956.
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