In French, when concluding a demonstration, we say "CQFD", which stands for "Ce Qu'il Fallait Démontrer".
Do English has an equivalent for this ?
Q.E.D - Quod erat demonstrandum - is put at the end of of a proof to signify that what we attempted to prove has been proven.
Q.E.F. - Quod erat faciendum - is a term that is used in geometric proof to signify that the geometric construction has been completed. It is a rarely used English abbreviation.
From the article the correct translation is Q.E.F. but from your description Q.E.D. sounds more accurate because it comes at the end of a proof/demonstration.
Note: From the comments below CQFD is always equivalent to QED.