Proof: By Euclid
(related to Proposition: 7.03: Greatest Common Divisor of Three Numbers)
- For let the greatest common measure, D, of the two (numbers) A and B have been taken [Prop. 7.2].
- So D either measures, or does not measure, C.
- First of all, let it measure (C).
- And it also measures A and B.
- Thus, D measures A, B, and C.
- Thus, D is a common measure of A, B, and C.
- So I say that (it is) also the greatest (common measure).
- For if D is not the greatest common measure of A, B, and C then some number greater than D will measure the numbers A, B, and C.
- Let it (so) measure (A, B, and C), and let it be E.
- Therefore, since E measures A, B, and C, it will thus also measure A and B.
- Thus, it will also measure the greatest common measure of A and B [Prop. 7.2 corr.] .
- And D is the greatest common measure of A and B.
- Thus, E measures D, the greater (measuring) the lesser.
- The very thing is impossible.
- Thus, some number which is greater than D cannot measure the numbers A, B, and C.
- Thus, D is the greatest common measure of A, B, and C.
- So let D not measure C.
- I say, first of all, that C and D are not prime to one another.
- For since A, B, C are not prime to one another, some number will measure them.
- So the (number) measuring A, B, and C will also measure A and B, and it will also measure the greatest common measure, D, of A and B [Prop. 7.2 corr.] .
- And it also measures C.
- Thus, some number will measure the numbers D and C.
- Thus, D and C are not prime to one another.
- Therefore, let their greatest common measure, E, have been taken [Prop. 7.2].
- And since E measures D, and D measures A and B, E thus also measures A and B.
- And it also measures C.
- Thus, E measures A, B, and C.
- Thus, E is a common measure of A, B, and C.
- So I say that (it is) also the greatest (common measure).
- For if E is not the greatest common measure of A, B, and C then some number greater than E will measure the numbers A, B, and C.
- Let it (so) measure (A, B, and C), and let it be F.
- And since F measures A, B, and C, it also measures A and B.
- Thus, it will also measure the greatest common measure of A and B [Prop. 7.2 corr.] .
- And D is the greatest common measure of A and B.
- Thus, F measures D.
- And it also measures C.
- Thus, F measures D and C.
- Thus, it will also measure the greatest common measure of D and C [Prop. 7.2 corr.] .
- And E is the greatest common measure of D and C.
- Thus, F measures E, the greater (measuring) the lesser.
- The very thing is impossible.
- Thus, some number which is greater than E does not measure the numbers A, B, and C.
- Thus, E is the greatest common measure of A, B, and C.
- (Which is) the very thing it was required to show.
∎
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References
Adapted from (subject to copyright, with kind permission)
- Fitzpatrick, Richard: Euclid's "Elements of Geometry"