When is the expression Q.E.D. used?

Date: 02/15/99 at 22:51:36
From: Andrea
Subject: When is the expression QED used?

I need to know when and why QED is used in a proof.  

Thanks for your help.

Date: 02/16/99 at 08:27:06
From: Doctor Rick
Subject: Re: When is the expression QED used?
  
Q.E.D. stands for the Latin "quod erat demonstrandum", which means 
"which was to be demonstrated (or proved)". It appears after the final 
step of the proof, which will be the statement that you originally set 
out to prove.

You will also find Q.E.F. at the end of a construction. This stands 
for "quod erat faciendum", "which was to be done (or made)".

For examples, see Euclid's Elements. His first proposition is a 
construction, and ends in Q.E.F.; in this online edition, notes after 
the proposition explain the use of Q.E.D. and Q.E.F.

  Euclid's Elements, Book I, Proposition 1 - David Joyce
  http://aleph0.clarku.edu/~djoyce/java/elements/bookI/propI1.html   

- Doctor Rick, The Math Forum
  http://mathforum.org/dr.math/