  A simple method to calculate the total annihilation cross section of massive
cold relics is presented. Exact expressions for $\arel$, $\brel$, and $\crel$
of $\sigma \vrel=\arel+\brel \vrel^2+\crel \vrel^4$ are obtained from simple
derivatives of the squared annihilation amplitude. Three examples are given. In
one example, the computed annihilation cross section of massive Dirac neutrinos
confirms recent correct calculations.
