**First**, Your last '1' needs to be a 'b', as some others brought up.

**Second**, I found Goal Seek really doesn't work too well (if you sub the 'b' in), because 1 will always solve this equation. This means that Goal Seek won't really solve it, since there's no way of constraining a minimum on the value, BUT if you want to just automate Goal Seeker, there is this expensive add-in.

**Third**, The VBA way to run GoalSeek is

Sheets("Sheet1").Range("A1").GoalSeek
Goal:=0,
ChangingCell:=Sheets("Sheet1").Range("B1")

where here "A1" would be the cell with the formula in it and "B1" would be the empty cell that would end up with the 'b' value. So you could create a loop for it and automate that way.

**Fourth**, The best way is to use Solver in VBA. Enable the Solver Add-In, then in VBA enable the Solver reference. Use the following code as an example, where the D column has the formula, the C column is where 'b' will end up, and $H$1 is the value 2, so that we keep it from solving it with b=1:

```
Public Sub SolveGeometric()
Dim i As Integer
For i = 2 To 3
SolverReset
SolverOk SetCell:="$D$" & i, MaxMinVal:=3, ValueOf:="0", _
ByChange:="$C$" & i
SolverAdd CellRef:="$C$" & i, Relation:=4 'keep it an integer
SolverAdd CellRef:="$C$" & i, Relation:=3, FormulaText:="$H$1"
' $H$1 = 2, so that it won't find 1 which always solves the equation
SolverSolve userFinish:=True
Next i
End Sub
```

`x = b^(d+1) - N(b-1) - 1`

that you need to solve to get a working value of`b`

for a known`x`

,`d`

and`N`

? What should the formula equal (ie, what is`x`

)? – DMA57361 Feb 5 '11 at 23:37`N = b + b^2 .. b^d`

and I need to find b. The above formula, when used correctly in Excel gives the correct value of b. – efficiencyIsBliss Feb 5 '11 at 23:42zero? If that's so, I think your formula may be incorrect -- when used with known values or`N`

,`d`

and`b`

(calculated from the original formula) the value returned isnotzero. Maybe`0 = b^(d+1) - N(b-1) - b`

instead? However, I wonder if there's some worth of first asking on Math.StackExchange how to get`b`

from the original formula for a known`N`

and`d`

, there might be a more mathematically direct approach than Excel goal seek (which is just an iterative search). – DMA57361 Feb 6 '11 at 0:1862 = summation n=1 to 5 b^n– wilson Feb 7 '11 at 6:55