Proof: By Euclid

For since $A$ and $B$ are prime to one another, and $A$ has made $C$ (by) multiplying itself, $C$ and $B$ are thus prime to one another [Prop. 7.25].
Therefore, since $C$ and $B$ are prime to one another, and $B$ has made $E$ (by) multiplying itself, $C$ and $E$ are thus prime to one another [Prop. 7.25].
Again, since $A$ and $B$ are prime to one another, and $B$ has made $E$ (by) multiplying itself, $A$ and $E$ are thus prime to one another [Prop. 7.25].
Therefore, since the two numbers $A$ and $C$ are both prime to each of the two numbers $B$ and $E$, the (number) created from (multiplying) $A$ and $C$ is thus prime to the (number created) from (multiplying) $B$ and $E$ [Prop. 7.26].
And $D$ is the (number created) from (multiplying) $A$ and $C$, and $F$ the (number created) from (multiplying) $B$ and $E$.
Thus, $D$ and $F$ are prime to one another.
(Which is) the very thing it was required to show.
∎

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