< Mentor Note -- thread moved to HH from the technical math forums, so no HH Template is shown > So the original problem was that a stationary hydrogen atom changed states from excited to lower state and emitted a photon, i solved for the energy of the photon hf taking into account the kinetic energy of the recoiling atom since the photon released and the atom recoiling have equal and opposite momenta. I came up with the formula, sorry i tried latexing everything but it wouldnt work for me: DeltaE=-pc+(p^2)/2m, both momentum p's are the atoms. The problem says I need to taylor expand this formula for small deltaE and keep the first two terms. I dont know how to taylor expand something random like this, a physics formula. Or it means taylor expand DeltaE=hf, but im pretty sure its the first one.... please help!