University Level Relativity: Relatavistic Decau Homework

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Homework Statement

University level question on relativity

Homework Equations

The Attempt at a Solution

momentum before is gamma*pionmass*0.93c
momentum after is gamma(u)*mass(u)*velocity(u) - E/c where E=pc which is the energy of massless particles

energy before is gamma*pionmass*c*c
energy after is gamma*mass(u)*c*c + pc

just wondering if what I've done above is correct as I've tried to do it that way and i get a really ridiculous answer. Any help is appreciated

 

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here is the question http://i45.tinypic.com/2it0cpy.jpg

 

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the answer is (masspion)^2 + (mass(u)^2)/ 2(masspion).. anyone know how they got to that, its 110 MeV, after some extreme calculation i got 103.75 MeV but i don't know how they got it simply by doing a simple calculation as above