| Line 12: | Line 12: | ||
<math>\int\frac{1}{1+e^x}\ dx = \int \frac{ e^x\,dx}{e^x(1+e^x)} = \int\frac{du}{(u-1)u}= | <math>\int\frac{1}{1+e^x}\ dx = \int \frac{ e^x\,dx}{e^x(1+e^x)} = \int\frac{du}{(u-1)u}= | ||
| − | \int\left[\frac{ | + | \int\left[\frac{1}{u-1}-\frac{1}{u}\right]\ du</math> |
| − | + | via integration by parts. | |
Revision as of 07:13, 30 September 2008
Evaluate the integral: $ \int \frac{dx}{1+e^x} $
I tried setting 'u' equal to '1+e^x' and 'du' equal to 'e^x dx'. Anyone know where to go from here? I'm sure there's some kind of manipulation that needs to be pulled off. I'll tell you if I figure it out.Gbrizend
Alright, I factored out a $ e^x $. I set 'u' equal to $ 1+\frac{1}{e^x} $ and 'du' to $ -1\frac{1}{e^x} dx $. The problem looks like this: $ \int \frac{dx}{e^x(1+\frac{1}{e^x})} $. Substitute with u and du. I got this answer: $ -Ln|1+\frac{1}{e^x}| + C $
Here's another way. Let $ u=1+e^x $. Then $ du=e^x dx $ and $ e^x=u-1 $. If I multiply and divide by $ e^x $ in order to get a $ du $ in the numerator, I get
$ \int\frac{1}{1+e^x}\ dx = \int \frac{ e^x\,dx}{e^x(1+e^x)} = \int\frac{du}{(u-1)u}= \int\left[\frac{1}{u-1}-\frac{1}{u}\right]\ du $
via integration by parts.
