Proportional Magnitudes have Proportional Remainders/Porism

Proof

 * Euclid-V-19.png

From :
 * $EB : FD = AB : CD$

From :
 * $AB : BE = CD : FD$

Therefore magnitudes compounded are proportional.

But it was also proved that:
 * $BA : AE = DC : CF$

and this is convertendo.