Hello, 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.