as promised here is the code revised to include square function

Sub coeffxi()
Dim i As Integer
Dim Prod_array(1 To 4)
Set Coeff = Range("c3:C6")
Set xi = Range("d3:d6")
Range("C8").Value = "Y ="
Range("D8").Value = Application.SumProduct(Coeff, xi)
For i = 1 To 4
Prod_array(i) = Coeff(i) * xi(i)
Next
Range("C9").Value = "SumSQ ="
Range("D9").Value = Application.SumSq(Prod_array)
End Sub

cheers,deano