[![problem question][1]][1]


  [1]: https://i.sstatic.net/9AUOQ.png

In the above problem, I've done part (a) by expanding the Lorentz factor as a power series and then truncating the "neglible" terms.

However, I'm stumped for deriving the "leading correcton". I've read the Wikipedia article on leading corrections:
https://en.wikipedia.org/wiki/Leading-order_term, but I still don't understand what the problem is asking for exactly?