can i put a micro atx motherboard in a dell optiplex 745 sff

Solution
While I can't say with certainty, the likelihood is pretty slim that an ATX board will fit correctly. Dell has been using the BTX form factor for years, so before you commit, if you can't otherwise find an answer, as a last resort I would open the Dell and measure the mounting holes, plus the back plate that covers the IO ports for the back of the motherboard. Also realize, BTX places the CPU in a different location than ATX for both acoustic and cooling purposes, so any cooling shrouds built in by Dell may be in the way of ATX components and need modification or removal.
While I can't say with certainty, the likelihood is pretty slim that an ATX board will fit correctly. Dell has been using the BTX form factor for years, so before you commit, if you can't otherwise find an answer, as a last resort I would open the Dell and measure the mounting holes, plus the back plate that covers the IO ports for the back of the motherboard. Also realize, BTX places the CPU in a different location than ATX for both acoustic and cooling purposes, so any cooling shrouds built in by Dell may be in the way of ATX components and need modification or removal.
 
Solution
you need to go to dell and see what board you got [form factor ] most times it can be a m-atx and atx is slim as in just a few models may have that

I also like how guys with a dell and I google -- dell optiplex 745 sff -- and dell don't show up just one link for a pdf that's it .. wheres the product support page ??? its kinda like you got this we got your money and now we forgot about you and our item ???

about as far as I got with it
http://www.dell.com/support/home/us/en/19/product-support/product/optiplex-745/research