In the attached image, I want to understand how to arrive at the equation 2.5.1, i.e. the variational expression. The problem is defined as best as it could and the further derivation follows smoothly.
The complete derivation is as shown in the image below:
I am interested in how the eq 2.1.1 leads to eq 2.5.1 ... or rather how does the variational operator (del) work?