Center of Momentum Energy problem

bweinert89

1. The problem statement, all variables and given/known data
Hi, I'm a bit confused about Center of Momentum Energy.

It has particles A and B collide to make C and D. It then asks to show that the CM energy of A is ((S+Ma^2-Mb^2)c^2)/(2Sqrt(S))


2. Relevant equations
The question gives the Mandelstam variables S=((Pa+Pb)^2)/c^2, t=((Pa-Pc)^2)/c^2, u=((Pa-Pd)^2)/c^2.


3. The attempt at a solution
I'm not quite sure what to do. I tried setting up the equation Ea+Eb = Ec+Ed where c and d only have resting energy. I square both sides and seperate try to get c*Sqrt((Ma^2)c^2+Pa^2) by itself since that is what the regular Ea equals but i dont get the desired outcome on the otherside.

-Ben