# An approximation for exponential.

1. Oct 10, 2009

### Alta

Is there an approximation for exp(-k*L) with L large but finite??

2. Oct 10, 2009

### Hurkyl

Staff Emeritus
Well, what is
$$\lim_{L \to +\infty} e^{-kL}$$​
? Doesn't calculus tell you that limit would be a good approximation?

(P.S. are you making any assumptions about k? )

3. Oct 10, 2009

### Alta

L is large but finite. No assumptions about k but it is positive.

4. Oct 10, 2009

### Delphi51

Power series. Use as many terms as you like to improve accuracy.
I can't seem to do Latex code, but you'll find it here about a screenfull down.
http://en.wikipedia.org/wiki/Power_series

Oops - no good for negative exponent!

5. Oct 10, 2009

### Alta

The trouble with that is that you can not say that exp(-k*L)=1+-k*L. you need the entire sum.