# Math Help - integrate (e^-x)/(1+e^-x)

1. ## integrate (e^-x)/(1+e^-x)

The answer is -ln(1+e^-x). I'm not real sure how it got there.

2. Are you familiar with $\int \frac {f'(x)}{f(x)} dx = \ln (f(x)) + C$?

3. Have you considered:

u = {whatever is in the denominator}

??

4. We havn't been taught that rule Matt.

And "u" substitution doesn't give me an ln. I doubt I'll be tested on this, but still wanted to know. Thanks Matt, the rule is something I need to know I guess.

5. Okay then, substitute $u = 1 + e^{-x}$. Get $du$ in terms of $x$ and $dx$ and see what happens.

6. Originally Posted by jebckr
We havn't been taught that rule Matt.

And "u" substitution doesn't give me an ln. I doubt I'll be tested on this, but still wanted to know. Thanks Matt, the rule is something I need to know I guess.
yes it does, you get $-\int \frac 1 u du$ which is wot?

7. ok, thanks. Got it.