Tracking changes to formula results in cells

worksheet or workbook? probably the easiest would be to run a macro on close that copies the data to another sheet. then compare that to new data after its opened.
something in the workbook module along the lines of

Private Sub Workbook_BeforeClose(Cancel As Boolean)
Sheets("sheet2").Range("a1:a3") = Sheets("sheet1").Range("a1:a3").Value

End Sub

probably easier would be to copy to the same sheet way off to the right then you could use conditional format quite easily to highlight changes

Private Sub Workbook_BeforeClose(Cancel As Boolean)
Sheets("sheet1").Range("aa1:aa3") = Sheets("sheet1").Range("a1:a3").Value

End Sub

alternatively use before save instead

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)