I guess my question is pretty basic, and following a procedure in the textbook by Lahiri and Pal. You can see the relevant pages at https://books.google.com/books?id=_UmPP8Yr5mYC&pg=PA245&source=gbs_toc_r&cad=4#v=onepage&q&f=false On eqs. (12.84)-(12.86). I don't see how to get from (12.85) to (12.86). How do you do this differentiation? Thanks.