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what it the problem with this method :<br />
=IIf([Forms]![invoicef]![r<wbr />ound]&lt;&gt;0,S<wbr />um([LineTo<wbr />tal]+[Form<wbr />s]![Invoic<wbr />eF]![round<wbr />]),Sum([Li<wbr />neTotal]))<wbr />
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what it the problem with this method :
=IIf([Forms]![invoicef]![round]<>0,Sum([LineTotal]+[Forms]![InvoiceF]![round]),Sum([LineTotal]))

a iif function together with a sum function don't work  in a form in access 2010

Microsoft Access is a rapid application development (RAD) relational database tool. Access can be used for both desktop and web-based applications, and uses VBA (Visual Basic for Applications) as its coding language.