Similar Plane Numbers have Same Ratio as between Two Squares
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Theorem
In the words of Euclid:
- Similar plane numbers have to one another the ratio which a square number has to a square number.
(The Elements: Book $\text{VIII}$: Proposition $26$)
Proof
Let $a$ and $b$ be similar plane numbers.
From Between two Similar Plane Numbers exists one Mean Proportional, there exists a mean proportional $m$ between them.
By definition of mean proportional:
- $\left({a, m, b}\right)$
is a geometric sequence.
From Form of Geometric Sequence of Integers:
- $\exists k, p, q \in Z: a = k p^2, b = k q^2, m = k p q$
Thus:
- $\dfrac a b = \dfrac {k p^2} {k q^2} = \dfrac {p^2} {q^2}$
Hence the result.
$\blacksquare$
Historical Note
This proof is Proposition $26$ of Book $\text{VIII}$ of Euclid's The Elements.
Sources
- 1926: Sir Thomas L. Heath: Euclid: The Thirteen Books of The Elements: Volume 2 (2nd ed.) ... (previous) ... (next): Book $\text{VIII}$. Propositions