Help with derivation of euler-lagrange equations

[teeeeee]

Hi,

I am trying to follow a derivation of the euler lagrange equations in one of my textbooks. It says that

[tex]\int ( f\frac{dL}{dx} + f'\frac{dL}{dx'}) dt[/tex]

=

[tex]f\frac{dL}{dx'} + \int f ( \frac{dL}{dx} - \frac{d}{dt}(\frac{dL}{dx'}) ) dt [/tex]

where f is an arbitrary function and L is the Lagrangian.I'm not sure how to perform this step. I think it has something to do with integration by parts but can't work it out. Any help would be appreciated.
Thanks
teeeeee

 

[CompuChip]

The integral over f (dL/dx) can be ignored, it simply sits in both expressions. So your question is, how do you get from

[tex]\int \frac{df}{dt} \frac{dL}{dx'}[/tex]
to
[tex]f \frac{dL}{dx'} - \int f \frac{d}{dt} \frac{dL}{dx'}[/tex]

right?
Because that is just partial integration in its purest form:
[tex]\int f' g = f g - \int f g'[/tex]
where g = dL/dx'.