June 25th, 2016, 05:57 PM  #1 
Senior Member Joined: May 2012 Posts: 205 Thanks: 5  Euler Lagrange
I'm looking at the EulerLagrange equation derivation, df/dyd/dx(df/dy')=0 In the derivation, they treat the the original integral, int(f(y',y,x)dx) like a regular calculus max/min problem, that is, they take the derivative with respect to some 'variance', while keeping the limits of the integral the same, and set it to zero, and from there make their way to the EulerLagrange equation... the goal being to find the conditions such that y(x) is the shortest path between two points. The variance is defined by a new function, Y(x) = y(x) + εg(x), where I take it that ε takes on arbitrary real numbers of multiplied by an arbitrary function g(x). I don't understand how this all really works and why we can treat the problem like a calculus problem, I don't even know enough to ask the right questions to help me understand it better.... What can I learn that will ultimately help me understand this? Last edited by skipjack; June 25th, 2016 at 11:53 PM. 
June 25th, 2016, 11:55 PM  #2 
Global Moderator Joined: Dec 2006 Posts: 20,833 Thanks: 2161 
What book or article are you using?

June 27th, 2016, 11:01 AM  #3 
Senior Member Joined: May 2012 Posts: 205 Thanks: 5 
This wasn't originally what I was looking at, but its the same derivation: Derivation of the EulerLagrangeEquation — Martin Ueding 

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euler, lagrange 
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