about mesh and normal

If i have a mesh,can i set some polygons of them just have flat eadges,which means some vertices shared in these polygons just have only normals,and some polygons of them are smooth eadges,which means shared vertices have be duplicated,they have own normals and texture coordinates??

If I understood you correctly, you should have a look at smoothing groups (http://www.youtube.com/watch?v=-Iswm1xKtGs)

So,what is this smoothing group used for?Exracting normals?or sth. else?!

its a way of calculating normals.

So, can u give me an example,code?It will be appreciated~

You can find example code from this link (which was recently posted in this forum)