If you can't reverse the formula, (I think you should), you can use this goalseeking function.
If you're new to VBA functions, follow the instructions in the text at the end
You can set up a table with r's in A1 and down, and use this formula in B1 and below:
=Backward(A1,0)
--
Kind regards,
Niek Otten
Function FindX(x As Double)
FindX = x * (1.24 + Application.WorksheetFunction.Ln(x))
End Function
' ===================================================================
Function Backward(ValueToBeFound As Double, MoreArguments As Double, _
Optional ReasonableGuess, Optional MaxNumberIters, _
Optional MaxDiffPerc) As Double
'
' Niek Otten, March 22 2006
'
' This EXAMPLE function goalseeks another function,
' called Forward. It works for almost any continuous function,
' although if that function has several maximum and/or minimum
' values, the value of the ReasonableGuess argument becomes
' important.
' It calculates the value for ReasonableGuess and for
' 1.2 * ReasonableGuess.
' It assumes that the function's graph is a straight line and
' extrapolates that line from these two values to find the value
' for the argument required to achieve ValueToBeFound.
' Of course that doesn't come out right, so it does it again for
' this new result and one of the other two results, depending on
' the required direction (greater or smaller).
' This process is repeated until the maximum number of calculations
' has been reached, in which case an errorvalue is returned,
' or until the value found is close enough, in which case
' the value of the most recently used argument is returned
Dim LowVar As Double, HighVar As Double, NowVar As Double
Dim LowResult As Double, HighResult As Double, NowResult As Double
Dim MaxDiff As Double
Dim NotReadyYet As Boolean
Dim IterCount As Long
If IsMissing(ReasonableGuess) Then ReasonableGuess = 1.5 ' use default Values
If IsMissing(MaxNumberIters) Then MaxNumberIters = 20 ' that make sense in the
If IsMissing(MaxDiffPerc) Then MaxDiffPerc = 0.001 ' context of the function
MaxDiff = ValueToBeFound * MaxDiffPerc
NotReadyYet = True
IterCount = 1
LowVar = ReasonableGuess
LowResult = Forward(LowVar, MoreArguments)
HighVar = LowVar * 1.2
HighResult = Forward(HighVar, MoreArguments)
While NotReadyYet
IterCount = IterCount + 1
If IterCount > MaxNumberIters Then
Backward = CVErr(xlErrValue) 'or some other errorvalue
Exit Function
End If
NowVar = ((ValueToBeFound - LowResult) * (HighVar - LowVar) + LowVar _
* (HighResult - LowResult)) / (HighResult - LowResult)
NowResult = Forward(NowVar, MoreArguments)
If NowResult > ValueToBeFound Then
HighVar = NowVar
HighResult = NowResult
Else
LowVar = NowVar
LowResult = NowResult
End If
If Abs(NowResult - ValueToBeFound) < MaxDiff Then NotReadyYet = False
Wend
Backward = NowVar
End Function
' ===================================================================
Function Forward(a As Double, b As Double) As Double
' This is just an example function;
' almost any continous function will work
Forward = FindX(a)
End Function
' ===================================================================
================================================
Pasting a User Defined Function (UDF)
Niek Otten, March 31, 2006
If you find a VBA function on the Internet or somebody mails you one, and you don't know how to implement it, follow these steps:
Select all the text of the function. CTRL+C (that is, press and hold down the CTRL key, press C, release both). This a shortcut
for Copy.
Go to Excel. Press ALT+F11 (same method: press and hold the ALT key, press the F11 key and release both). You are now in the
Visual Basic Editor (VBE).
From the menu bar, choose Insert>Module. There should now be a blank module sheet in front of you. Click in it and then press
CTRL+V (same method.). This a shortcut for Paste. You should now see the text of the function in the Module.
Press ALT+F11 again to return to your Excel worksheet.
You should now be able to use the function as if it were a built-in function of Excel, like =SUM(..)
================================================
