# Gamma ray energies from an electron annihilation?

I recently started studying Special Relativity for fun through reading the classic book Spacetime Physics. I came across a practice problem that I have difficult understanding:

An electron moving with some kinetic energy K collides with another electron to produce 2 gamma rays travelling at different angles with respect to the moving electron. Determine an expression for the energies as a function of the electron mass, energy, and angle of outgoing rays.

I'm not quite sure how to set up the conservation momentum equations to solve for this...?

• actually you likely need conservation of the net momentum 4-vector. Presumably the second electron is at rest? – ZeroTheHero Mar 2 '17 at 3:26
• Yes the second electron is at rest – stanfordude Mar 2 '17 at 3:31
• I'm not too familiar with the mathematics involved with 4-momentum, but any help would be appreciated! – stanfordude Mar 2 '17 at 3:33
• – ZeroTheHero Mar 2 '17 at 3:36
• is it possible you are misquoting and it is an electron hitting a positron? Seems to me an undefined problem . – anna v Mar 2 '17 at 5:40