# Corollary: Cor. 10.006: Magnitudes with Rational Ratio are Commensurable

### (Corollary to Proposition 6 from Book 10 of Euclid's “Elements”)

So it is clear, from this, that if there are two numbers, like $D$ and $E$, and a straight line, like $A$, then it is possible to contrive that as the number $D$ (is) to the number $E$, so the straight line (is) to (another) straight line (i.e., $F$). And if the mean proportion3, (say) $B$, is taken of $A$ and $F$, then as $A$ is to $F$, so the (square) on $A$ (will be) to the (square) on $B$. That is to say, as the first (is) to the third, so the (figure) on the first (is) to the similar, and similarly described, (figure) on the second [Prop. 6.19 corr.] 4. But, as $A$ (is) to $F$, so the number $D$ is to the number $E$. Thus, it has also been contrived that as the number $D$ (is) to the number $E$, so the (figure) on the straight line $A$ (is) to the (similar figure) on the straight line $B$. (Which is) the very thing it was required to show.

### Modern Formulation

(not yet contributed)

Proofs: 1

Proofs: 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Propositions: 15 16 17 18 19 20 21 22 23 24 25

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