An $\eta$ meson decays into $2 \gamma$ while moving in the $x$ direction with energy $E_\eta=5 \mathrm{GeV}$.
1. If the two $\gamma \mathrm{s}$ are emitted in the $+x$ and $-x$ directions, what are their energies?
2. If the two $\gamma \mathrm{s}$ are emitted at equal and opposite angles $\pm \theta$ with $x$, what is the angle between the two?