Integrating 1/(1+e^x)

[MaximumTaco]

Integrating 1/(1+e^x):

I am currently substituting 1+e^x = u,

S (u*e^x)^-1 du since du/dx = e^x

S (u*(u-1))^-1 du (S represents the integral symbol)

But i looked up the answer and got Ln (e^x(1+e^x)) +c

i need to understand how this is done, thanks in advance everyone.

[Mulder]

Check out this thread (http://www.physicsforums.com/showthread.php?s=&threadid=3507) in Homework Help.

[HallsofIvy]

After you have integral (u(u-1))^(-1) du= integral 1/(u(u-1)) du

you can use "partial fractions", rewriting

1/(u(u-1))= A/u+ B/(u-1) for proper A and B. Then integrate

integral (A/u)du+ integral B/(u-1) du.

[fmadero]

Guys this problem is a lot easier than it looks. From a quote from a PHD Student at Texas Tech Univ. Krishna Kaphle "Adding 0 or Multiplying By 1 is Free In Math".

What does this mean for this problem well lets rewrite our expression
1. \int{\frac{1+e^x-e^x}{1+e^x}dx} [lets add 1 to our expression]

2. \int{\frac{[1+e^x]-e^x}{1+e^x}dx} [now focus in on the numerator see whats in brackets matches the denominator]

3. \int{\frac{1+e^x}{1+e^x}dx}-\int{\frac{e^x}{1+e^x}dx} [now lets split the fraction]

4. \int{1dx}-\int{\frac{e^x}{1+e^x}dx} [now just use substiution method for right hand integral. u=1+e^x, du = e^xdx]

5. \int{1dx}-\int{\frac{du}{u}}

6. x-ln|{u}|+C

7. x-ln|{1+e^x}|+C [substitue u back]

If you don't believe this or too lazy to do its derivative use this tool:
http://library.wolfram.com/webMathematica/Education/WalkD.jsp
and enter this expression: x-ln[1+e^x]

kiko

[adriank]

If the poster above can bump this thread, so can I. Besides, I think my solution is the nicest one between this thread and the thread referred to in post #2. :)

I make the substitution u = e^{-x} + 1:

\int \frac{1}{1 + e^x} \,dx
= \int \frac{e^{-x}}{e^{-x} + 1} \,dx
= \int -\frac{1}{u} \,du
= -\ln \lvert u \rvert + C
= -\ln(e^{-x} + 1) + C.


Note that
-\ln(e^{-x} + 1) = x - \ln(1 + e^x) = \ln \left( \frac{e^x}{1 + e^x} \right).


edit: Isn't this thread supposed to be locked, being archived?