= Angular relation for two-photon electron-positron annihilation
{title2=$\frac{1+\cos(\theta_1+\theta_2)}{\cos\theta_1+\cos\theta_2}=\sqrt{\frac{\gamma-1}{\gamma+1}}$}
When an electron of Lorentz factor $\gamma>1$ annihilates with a stationary equal-mass positron, transverse <four-momentum conservation> makes the <photons>' transverse components opposite. Eliminating their positive frequencies gives the displayed relation for noncollinear <photon> angles to the incoming electron. The exactly collinear forward/backward configuration is physically allowed but makes the quotient $0/0$; its undivided cross-multiplied identity remains valid. The angle relation thus requires nondegeneracy or an appropriate limit, rather than an assertion that every allowed angle pair defines the quotient.
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