Centre of mass — UPSC CSE 1991 Physics PYQ

Prove that if E and E' are respectively the neutron energies in the laboratory system, before and after collision with a nucleus of mass number A. then \frac{E'}{E} = \frac{1 + A^2 + 2 A \mu_e}{(1 + A)^2} where \mu_e is the cosine of the scattering angle in the centre of mass system.