Construction of Third Proportional Straight Line
Theorem
Given any two straight lines of given length $a$ and $b$, it is possible to construct a third straight line of length $c$ such that $a : b = b : c$.
In the words of Euclid:
- To two given straight lines to find a third proportional.
(The Elements: Book $\text{VI}$: Proposition $11$)
Construction
Let $AB, AC$ be the two given straight lines.
Let them be placed to contain any angle.
Let $AB$ be produced to $D$, and $AC$ be produced to $E$.
Let $BD$ be constructed equal to $AC$.
Join $BC$ and construct $DE$ parallel to $BC$.
Then $CE$ is the required third proportional line.
Proof
We have that:
- $BC \parallel DE$
So from Parallel Transversal Theorem:
- $AB : BD = AC : CE$
But $BD = AC$ and so:
- $AB : AC = AC : CE$
as required.
$\blacksquare$
Historical Note
This proof is Proposition $11$ of Book $\text{VI}$ of Euclid's The Elements.
It is a special case of Proposition $12$ of Book $\text{VI} $: Construction of Fourth Proportional Straight Line.
Sources
- 1926: Sir Thomas L. Heath: Euclid: The Thirteen Books of The Elements: Volume 2 (2nd ed.) ... (previous) ... (next): Book $\text{VI}$. Propositions