Feynman Lectures Volume 3 Equation 9.41

[vappole]

I don't understand how the last equation in the attached picture is derived from equations 9.40 and 9.39, can anyone help?

 

[Simon Bridge]

Do the substitution as described and use the fact that $\mu\epsilon$ is small compared with $A$.
The last equation is for the LHS only of eq.9.39 ... where $C_I$ is given in the first equation in 9.40.

 

[vappole]

Oh, I got it. Tx

 

[Simon Bridge]

No worries - the RHS in the last equ. looks a lot like the RHS in equ.9.39 so I expect that's what threw you. I suspect that Feynman exploits the similarity on the next page.