How to sum cells and include previous sum?

Question

I want to add two cell values.

Add column E and Column F values into Colum D:

[1

If I delete those values it should not affect the resultant cell.
If I change the value or if I add new values in those cells the values should add to the previous resultant cell.

E.g. Cell A1=6 and cell A2=4. Their sum in cell A3 will be 10.
If I change the values of both cells to A1=8 and A2=5 then A3 should be 23.

Answer 1

The simple code:

Private Sub Worksheet_Change(ByVal Target As Range)
  If Target.Address <> [A1].Address And Target.Address <> [A2].Address Then Exit Sub
  If IsEmpty(Target) Or Not IsNumeric(Target) Then Exit Sub
  [A3] = [A3] + Target.Value
End Sub

This picture should help, I guess. Use the latest code, please.

If you need to sum 3 cells, e. g. A4 = A1 + A2 + A3:

Private Sub Worksheet_Change(ByVal Target As Range)
  If Target.Address <> [A1].Address And Target.Address <> [A2].Address _ 
    And Target.Address <> [A3].Address Then Exit Sub
  If IsEmpty(Target) Or Not IsNumeric(Target) Then Exit Sub
  [A4] = [A4] + Target.Value
End Sub

If you need to have 2 triplets of cells, e. g. A3 = A1 + A2 and B3 = B1 + B2:

Private Sub Worksheet_Change(ByVal Target As Range)
  If Target.Row > 2 Then Exit Sub
  If IsEmpty(Target) Or Not IsNumeric(Target) Then Exit Sub
  If Target.Column = 1 Then
    [A3] = [A3] + Target.Value
  ElseIf Target.Column = 2 Then
    [B3] = [B3] + Target.Value
  End If
End Sub