A particle called neutral pi meson (or pion) decays into two photons and nothing else. If the pion is initially at rest (rest mass 135Mev).

(a) what are the energies of the photons ?
(b) What are the magnitudes of the momenta of the photons
(c) How are they moving with respect to each other.

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See here:

http://www.jiskha.com/display.cgi?id=1307471410