= Solution
Work in the parent's rest frame. <Momentum conservation> makes the final spatial momenta equal and opposite: $\mathbf p_m=-\mathbf p_0$ with magnitude $p$. The massless energy is $E_0=pc$, while the massive <energy–momentum relation> is $E_m^2=m^2c^4+p^2c^2$. <Conservation of energy> gives $E_m+pc=Mc^2$. Squaring $E_m=Mc^2-pc$ and cancelling the $p^2c^2$ terms gives
$$
\boxed{p=\frac{M^2-m^2}{2M}c,\qquad E_0=\frac{M^2-m^2}{2M}c^2,\qquad E_m=\frac{M^2+m^2}{2M}c^2.}
$$
The two directions are arbitrary but opposite in this frame. These are total energies, including the massive daughter's rest energy, as required for <relativistic two-body decay>.
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