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## Homework Statement

An electron and a positron are moving side by side in the +x direction at 0.50c when they annihilate each other creating two gamma rays. What is the energy of each photon?

## Homework Equations

(mc

^{2}+ K

_{+}) + (mc

^{2}+ K

_{-}) = E

_{1}+ E

_{2}

## The Attempt at a Solution

I solved out that K = mc

^{2}= 0.511MeV ( [tex]\gamma[/tex] - 1 ) where [tex]\gamma[/tex] = 1.155. Therefore K should equal 79.1keV. Plug this into the equation up in that "relevant equations" section and I get E

_{1}+ E

_{2}= 1.18MeV.

I know the answer is E

_{1}= .885 MeV in the +x direction and E

_{2}= .295 MeV in the -x direction as my prof gave us the answers. These values verify that the calculations I performed earlier are correct.

However, I don't know how to obtain the answers. It seems like it should be stupidly simple, but I just can't figure it out. I tried using hc/[tex]\lambda[/tex] = E to solve for wavelength and then tried plugging stuff in again to get energy but that doesn't work. And even if it did, how would I know direction? If anyone can just give me that slight push in the right direction I'd be very grateful!