Possible Duplicate:
How can I split an equation over two lines

I have a displaymath formula that's too long. \linebreak and other commands that work in inline math don't seem to do anything. Is there any way to break the displaymath onto multiple lines? (or preferably, have latex break it automatically?)

I saw some other related questions here at tex.stackexchange, which said "use align/eqnarray*/etc instead". My formula doesn't need breaking for layout reasons, just because it's too long, so I'd prefer to keep using displaymath. Is there a way to linebreak without changing environments?