# Calculus of variations

Hello, here is my problem.[/PLAIN] [Broken]

http://imgur.com/VAu2sXl

My confusion lies in, why those particular partial derivatives are chosen to be acted upon the auxiliary function and then how they are put together to get the Euler-Lagrange equation?

My guess is that it's related to the turning points of the auxiliary equation, however i don't know why those derivatives are chosen and how they're related to the EL equation.

Sam

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## Answers and Replies

Ssnow
Gold Member
You must interpret ## \int_{a}^{b} 12x\cdot y(x)+\left(\frac{\partial}{\partial x} y(x)\right)^{2}dx## as your action functional ##S(t)## where the time is represented by ##t=x, q(t)=y(x), \frac{\partial}{\partial x} y(x)=\dot{q}(t)## and ##f(x,y,y')## is the lagrangian ##\mathcal{L}(t,q,\dot{q})##. In the example derivatives are alculated in order to write the Euler Lagrange equation:

## \frac{\partial}{\partial q}\mathcal{L}-\frac{d}{dt}\frac{\partial}{\partial \dot{q}}\mathcal{L}=\frac{\partial}{\partial y}f-\frac{d}{dx}\frac{\partial}{\partial z}f=0##

They used with abuse of notation ##\frac{\partial}{\partial x}=\frac{d}{dx}## so you solve the Euler Lagrange equation and obtain the extremal function ##y(x)##...

remember ##z=y'##, I hope in a clarification ...

Oh! Thanks, that's clarified the technique, I can do the rest of worksheet now.