I am in the process of writing VBA code to perform Lagrang
interpolation and I am not sure how to write the code for the Lagrang
polynomial function. This function is in the attachment labele
"LIP.xls". The problem is that this function involves a summation an
a product summation and I am not sure how to handle this in VBA code.
My spreadsheet is attached as "exp.xls". In this spreadsheet, the use
may input as many data points (x and y values) as they wish. Then the
can input an x value and the macro should calculate the interpolated
value.

For the values currently in the spreadsheet (x = 3);
y = 493.84

NOTE: Even though the spreadsheet currently contains only three dat
points, the user may input as many data points as they wish.

The excel spreadsheet attached gives an example of how the Lagrang
polynomial should be calculated for the values currently in th
spreadsheet.

Here is the code I have written so far

Private Sub Worksheet_Change(ByVal Target As Range)

Dim xn, yn, x
If Target.Address = "$D$5" Then

xn = Application.WorksheetFunction.CountA(Columns("A:A" )) - 3
yn = Application.WorksheetFunction.CountA(Columns("B:B" )) - 1

If xn < yn Then
MsgBox ("There must be the same number of x's as y's"), , "Hold Up!"
Exit Sub
End If

x = Range("D5").Value

????????????What should go here?????????????

End If

End Sub

How may I write a VBA macro to perform this task?

Please Help!
Thanks

Attachment filename: lip.xls
Download attachment: http://www.excelforum.com/attachment.php?postid=54635
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