Answer
See answers.
Work Step by Step
The initial energy is supplied by the 2 incoming particles. Equation 26–6b tells us the initial energies. Let M be the mass of the final particle; the final energy is the rest energy of this particle since there is no KE.
$$2\gamma mc^2=Mc^2$$
$$M=2\gamma m=\frac{2m}{\sqrt{1-v^2/c^2}}$$
Energy is conserved. There was no loss of energy in the collision (though the KE was transformed into mass).
There is no final kinetic energy. All of the kinetic energy was lost.
$$KE_{lost}=KE_{initial}=2(\gamma-1)mc^2=(\frac{1}{\sqrt{1-v^2/c^2}}-1)2mc^2$$
