Read Appendix 2 in the attached link.....The proof you requested!
If you cannot or have trouble understanding it just ask, I am sure there a plenty of people that will help you out!
https://hotcopper.com.au/documentdownload?id=uOMxKKzFkiWRTLKhOROKAxjvTDYD4Qy5zBCZtvJ6ke92GA==