# integrate (f*g')/g

• Feb 7th 2010, 05:24 AM
factfinder
integrate (f*g')/g
Hi all
Are there common ways to manipulate the integral

$\int\frac{fg'}{g}dx$

so that it can be evaluated? would one need to know the exact functions in question or can this be answered generally?
• Feb 7th 2010, 05:44 AM
Moo
Hello,

Nope it can't be solved without further information. It's not a known form :)
• Feb 7th 2010, 09:15 AM
chisigma
Calling $x$ the independent variable is...

$\frac{g^{'}(x)}{g(x)}= \frac{d}{dx} \ln g(x)$ (1)

... so that integrating by parts we have the identity...

$\int f(x)\cdot \frac{g^{'}(x)}{g(x)}\cdot dx = f(x)\cdot \ln g(x) - \int f^{'}(x)\cdot \ln g(x)\cdot dx$ (2)

Honestly I don't know if (2) can be of some utility for You (Thinking)...

Kind regards

$\chi$ $\sigma$
• Feb 8th 2010, 12:05 AM
chisigma
An interesting possibility is when is $f(x)= g(x) + \chi$ with $\chi$ an arbitrary constant, so that is $f^{'} (x) = g^{'} (x)$. In such a case is...

$\int f(x)\cdot \frac {g^{'} (x)}{g(x)}\cdot dx= 2\cdot g(x)\cdot \ln g(x) + (\chi -1)\cdot g(x) + c$

Kind regards

$\chi$ $\sigma$