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I recently began an Analytical Mechanics course and find it impossible to understand how to derive Euler-Lagrange formula from the principle of least action.

There is actually one step I can't understand, how we get from:

I WOULD HAVE USED LaTeX HERE BUT IT'S NOT POSSIBLE TO SEE IT IN "PREVIEW POST"!

I DON'T TRUST I CAN SUBMIT WITHOUT ERRORS.

Can anyone look at the image and explain how they got to the expression below? and I'm not sure I know what exactly is the meaning of δ (delta that is)...

Thanks in advance.

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# Homework Help: Action (and the principle of least of which)

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