Commensurability of Squares on Proportional Straight Lines
Theorem
In the words of Euclid:
- If two straight lines be proportional, and the square on the first be greater than the square on the second by the square on a straight line commensurable with the first, the square on the third will also be greater than the square on the fourth by the square on a straight line commensurable with the third.
And, if the square on the first be greater than the square on the second by the square on a straight line incommensurable with the first, the square on the third will also be greater than the square on the fourth by the square on a straight line incommensurable with the third.
(The Elements: Book $\text{X}$: Proposition $14$)
Lemma
In the words of Euclid:
- Given two unequal straight lines, to find by what square the square on the greater is greater than the square on the less.
(The Elements: Book $\text{X}$: Proposition $14$ : Lemma)
Proof
Let $A$, $B$, $C$ and $D$ be four straight lines in proportion, such that:
- $A : B = C : D$
Using the lemma let the straight lines $E$ and $F$ be found such that:
- $A^2 = B^2 + E^2$
- $C^2 = D^2 + F^2$
As $A : B = C : D$ it follows from Similar Figures on Proportional Straight Lines that:
- $A^2 : B^2 = C^2 : D^2$
But:
- $E^2 + B^2 = A^2$
- $F^2 + D^2 = C^2$
Therefore:
- $E^2 + B^2 : B^2 = F^2 + D^2 : D^2$
and so by Magnitudes Proportional Compounded are Proportional Separated:
- $E^2 : B^2 = F^2 : D^2$
By Similar Figures on Proportional Straight Lines:
- $B : E = D : F$
But:
- $A : B = C : D$
Therefore from Equality of Ratios Ex Aequali:
- $A : E = C : F$
From Commensurability of Elements of Proportional Magnitudes it follows that:
- if $A$ is commensurable with $E$ then $C$ is also commensurable with $F$
and:
- if $A$ is incommensurable with $E$ then $C$ is also incommensurable with $F$.
$\blacksquare$
Historical Note
This proof is Proposition $14$ of Book $\text{X}$ of Euclid's The Elements.
Sources
- 1926: Sir Thomas L. Heath: Euclid: The Thirteen Books of The Elements: Volume 3 (2nd ed.) ... (previous) ... (next): Book $\text{X}$. Propositions