The log of a negative number will always be complex:

IMLN() for the natural log
IMLOG2() for log based 2
IMLOG10() for log based 10

if you are trying to raise e to a power, then use EXP()
Say A1 contains t
A2 contains T
A3 contains U
A4 contains the formula =A3*(1-exp(-A1/A2))

Here is an example A1 thru A4:

5
7
11
5.615041745


--
Gary's Student


"Craig" wrote:

im looking for the natural log (NL) or LOGe or e, however you choose to write
it. the exact formulae/equation i wish to enter is Um=U((1-e(-t/T)).
unfortunatley Excel doesn't like it, even when brocken down. could you help
further?

"Gary''s Student" wrote:

Your problem is imaginary:
=IMLOG2("-1+0i")
will return
4.53236014217602i

--
Gary''s Student


"Craig" wrote:

im trying to use negative numbers with excel's natural log function, and have
found it doesnt work the way im trying it anyway, has anyone out there used
negative numbers with Excel's natural log function and any sucess?