1) Write down general expression for the decay rate a -> 1+2.
   Outline steps taken how from the starting formula of Fermi's golden rule
   one arrives to the final expression.

2) Write down expression for the differential cross-section
   for the process a+b -> c+d in the centre-of-mass frame.
   Outline steps taken how from the starting formula of Fermi's golden rule
   and general expression for the decay rates  one arrives to the
   final expression.