Break equation

[Beaubert Francois 0251858286]

I would like to automaticaly break too long equations (that are wider than the page)
I have heard about a package called breqn.sty but it doesn't seem to work properly

I their a simple way to do this

-- 
                              """
                              o o
+-----------------------ooO---(_)---Ooo---------------------------+
|                                                                 |
|                      François Beaubert                          |
|                   Ecole des Mines de Nantes                     |
|  4 rue Alfred Kastler BP 20722 44307 Nantes Cedex 3 France      |
| Telephone : (0/33)2.51.85.82.86  Telecopie : (0/33)2.51.85.82.99|
|                   [EMAIL PROTECTED]                      |
+-----------------------------------------------------------------+