Proportional Numbers have Proportional Differences

Theorem
That is:
 * $a : b = c : d \implies \left({a - c}\right) : \left({b - d}\right) = a : b$

where $a : b$ denotes the ratio of $a$ to $b$.

Proof
As the whole $AB$ is to the whole $CD$, so let the $AE$ subtracted be to $CF$ subtracted.

We need to show that $EB : FD = AB : CD$.


 * Euclid-VII-11.png

We have that :$AB : CD = AE : CF$.

So from we have that whatever part or parts $AB$ is of $CD$, the same part or parts is $AE$ of $CF$.

So from:

and:

$EB$ is the same part or parts of $FD$ that $AB$ is of $CD$.

So by $EB : FD = AB : CD$.