【正文】 .5 .............................................................................................................5....................................................................................................................................5 ....................................................................................................................................5 、假定和限制............................................................................................................5 ....................................................................................................5 ............................................................................................................................5 .............................................................................................................5........................................................................................................5 ............................................................................................................................6 ............................................................................................................................6 ....................................................................................................................................6 ....................................................................................................................................6 ................................................................................................................................6 ....................................................................................................................6........................................................................................................6 ........................................................................................................6 ............................................................................................................................6 ....................................................................................................................................6 ................................................................................................................................7 ....................................................................................................7 ......................................................................................................7............................................................................................................7 ............................................................................................................7 .................................................................................................................7....................................................................................................................................7 ....................................................................................................................................8 ............................................................................................................8 ............................................................................................................8 ...................................................................................................................................8 、假定和限制