text: Note that
  where m is the mass of particle i.
  To prove this, we need to use two facts:
  :*The square of a particle's four momentum is the square of its mass,
  :*And conservation of four-momentum,
  So, to begin,
  Then adding the three while inserting squared masses leads to,
  Then note that the last four terms add up to zero using conservation of four-momentum,
  So finally,