Exponential function-Solution

Hi,

Put a value of 1 in A1.

Put this formula in B1:

=SUMPRODUCT(A1^-ROW(INDIRECT("18:48")))

Then use Solver with:

Set Objective: $B$1
To: Value Of: 1
By Changing Variable Cells: $A$1

I get a value of 1.127278728.

Regards

XOR LX has proposed the solution I would use. The only caution I would add to his solution is that you may need to take careful consideration of round-off error. If you have ever seen any discussion on summing a harmonic series like this, you have probably seen that you should structure the addition to work from small to large rather than from large to small (example discussion here: http://stackoverflow.com/questions/7...g-harmonic-sum). I used the SERIESSUM() function instead of SUMPRODUCT() (should not matter which one you use). Using XOR LX's results of 1.1272..., when I add it up from -18 to -48, I get 1.0028. When I sum it up from -48 to -18, I get 1.1304. In a case like this, you need to pay attention to round off error or you will get a wrong (or less accurate) result.