how do i use excel's natural log function with negative numbers?

[Guest]

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?

 

[Dana DeLouis]

Hi. If I understand the question, the log of say -5 would be a complex
number.
1.609+3.14159 i

If A1 holds -5, then perhaps two ways...

=IF(A1<0,COMPLEX(LOG(ABS(A1)),PI()),LOG(A1))

or more simply the engineering function...
=IMLN(A1)

 

[Guest]

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

 

[Guest]

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?

 

[Guest]

Dana Hi, this was of some help but a little hard for me to understand, here
is the exact equation i wish to put into excel - Um = U(1-e(-t-T)), in my
cells i would have for example A1 = -t, U is constant & T is constant. could
you help further. i believe the reason the trouble occurs is also because im
not using LOG im using Natural LOG (NL).

 

[Dana DeLouis]

= U(1-e(-t-T)),

Hi. I don't really follow. I believe the above equation might be written
like this.
Change 't' and 'T' to your cell references.

=U*(1-EXP(-t-T))

Please post back with any further questions. :>)

 

[Dana DeLouis]

Hi. I took e(-t-T) to mean E^(-t-T)

If you really meant the natural log, then perhaps...

=U*(1-LN(-t-T))

or perhaps w/ cell references: =U*(1-LN(-A1-A2))

Again, I'm not sure I follow, so I'm just throwing this out as an idea.

 

[Guest]

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