extermising functional

Hi everyone,

I have a functional "L" which I want to solve Euler-Lagrange for it and then extermise that functionalexfun.mw by its equation of motions. I know the result must be same to a function like "A" but I cant obtain that. "a", "b" and "s" are constants and not important, I insist more on the shape of "A"

Thanks a lot