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?