The question is:
A pion decays into a muon and a neutrino (which is almost massless, so we’ll take it to have a mass of zero). If the pion is initially at rest, find the energy of the outgoing muon.
Final Answer: $$E_μ=(m_π^2+m_μ^2)c^2/2m_π$$
I used conservation of energy and so the initial energy is just the rest energy of the pion which equals $m_πc^2$ and this should equal the final energy.
Getting the final energy is what I am having trouble with, the mass of the neutrino is approx. 0 so I used the $$E=\sqrt{p^2c^2 + m^2c^4}$$ and crossed out the term with m leaving $E_\nu=pc$. Then, for the muon there is rest energy and kinetic energy I eventually get $$E_μ=m_μc^2/\sqrt{1-u_μ^2/c^2}.$$
However, the final answer neither includes a $p$ nor a $u$, so I think that the problem is in finding the final energy and isolating for the $E_μ$.