Coprime Numbers form Fraction in Lowest Terms

Proof
Let $A, B$ be (natural) numbers which are prime to one another.

We need to show that $A$ and $B$ are the least of those which have the same ratio with them.


 * Euclid-VII-21.png

Suppose this were not the case.

Then there would be some numbers $C, D$ which are less than $A, B$ such that $A : B = C : D$.

From we have that $C$ measures $A$ the same number of times that $D$ measures $B$.

Now, as many times as $C$ measures $A$, let there be so many units in $E$.

Then from $E$ also measures $A$ according to the units in $C$.

For the same reason $E$ also measures $B$ according to the units in $D$.

Therefore $E$ measures $A$ and $B$, which are prime to one another, which is contrary to our initial hypothesis.

Hence the result.