  We present a simple derivation of the energy formula found by Tan, relative
to the single channel hamiltonian relevant for ultracold Fermi gases. This
derivation is generalized to particles with different masses, to arbitrary
mixtures, and to two-dimensional space. We show how, in a field theoretical
approach, the $1/k^4$ tail in the momentum distribution and the energy formula
arise in a natural way. As a specific example, we consider quantitative
calculations of the energy, from different formulas within the ladder diagrams
approximation in the normal state. The comparison of the results provides an
indication on the quality of the approximation.
