- #1

offscene

- 7

- 2

- Homework Statement
- Not exactly homework but I was reading through the book "QFT for the gifted amateur by Lancaster and Blundell" and I was confused about how the line just above equation 1.33 is derived (Image attached below).

- Relevant Equations
- Euler Lagrange equation, the principle of least action.

I'm confused on how to derive the multidimensional generalization for a multivariable function. Everything makes sense here except the line,

$$

\frac{\delta S}{\delta \psi} = \frac{\partial L}{\partial \psi} - \frac{d}{dx} \frac{\partial L}{\partial(\frac{\partial \psi}{\partial x})} - \frac{d}{dt} \frac{\partial L}{\partial(\frac{\partial \psi}{\partial t})}

$$ and I'm confused about which rule I can use to derive this form of the Euler-Lagrange equation.