Homework Help: Need a little help starting this integral

1. Feb 12, 2006

silverdiesel

$$\int\frac{1}{e^x+1}dx$$

I have tried substitution, partial fraction decomposition, and integration by parts but I cant seem to figure it out. Any help is much appriciated.

2. Feb 12, 2006

Physics Monkey

Hint: multiply top and bottom by $$e^{-x}$$.

3. Feb 12, 2006

silverdiesel

okay, then I can use u sub, which nets...

$$-ln (1+e^{-x})$$

is that right?

Last edited: Feb 12, 2006
4. Feb 12, 2006

silverdiesel

That does not seem right. What I have I done wrong?

$$\int\frac{e^{-x}}{1+e^{-x}}dx$$

$$u = 1+e^{-x}\\-du = e^{-x}$$

$$-\int\frac{1}{u}du$$

Last edited: Feb 12, 2006
5. Feb 12, 2006

d_leet

It looks right to me, why do you think it's wrong?

6. Feb 12, 2006

silverdiesel

After further review, I dont think its wrong. I guess I am just working on building confidence in the integration process and I was hoping for some confirmation that is was done right. Thank you. These forums have been extremely helpful and I really appriciate all the quick and intelligent responces.