Elastic lepton scattering (8.1) can be regarded as a special case of inelastic scattering (8.26) in which the final hadronic state X is a single proton. Find the corresponding value of the scaling variable (8.29). Hence show that in the rest frame of the proton, the initial (E) and final (E') lepton energies are related by 

M(E – E') = EE'(1 − cosθ),

where θ is the lepton scattering angle, M is the proton mass and lepton masses are assumed to be negligible compared with their energies.