Hi,
The proof is from Trench's Introduction to Real Analysis, which can be freely downloaded here
William Trench - Trinity University Mathematics
I was wary of posting the entire proof verbatim in case of any infringement (not sure what the laws are concerning mathematical proofs but I thought it better to be safe).
The proof in question is on page 78 of the book.
Thanks,
Stonehambey
EDIT: Actually I believe there may be a bracket missing from equation (10) in the proof, immediately after
. Can anyone confirm this?