It's solution as followings,
I choose units such that c = 1, and assume that me = 0.511MeV.
Since the pion is at rest conservation of momentum dictates that the momenta of the
muon and the neutrino be equal in magnitude (and opposite in direction),
p μ = p ν . (A)
Since the pion is at rest its energy equals its mass, Eπ = mπ . Since the neutrino is
massless its energy equals its momentum, E ν = pν . By conservation of energy,
Eπ = E μ + Eν , so
Eμ = mπ -pν. (B)
Substituting the right sides of (A) and (B) into the left side of the fundamental
kinematic equation for the muon (Eμ2-pμ2)= mμ2 yields
Solving for pν gives (the magnitudes of) the momenta of the decay particles and the
kinetic energy (equal to the total energy) of the massless neutrino,
The kinetic energy of the muon equals its total energy minus its mass which, using (B),
is (mπ-pν)-mν=4.08 MeV . Q.E.D.
Physics is totally different from Math, I think.
Math is just a tool for Physics.