A particle at rest decays into 2 particles

[ChronicApathy]

Homework Statement

A particle at rest with mass M decays into 2 particles, one with mass m1 and the other with mass m2. Use conservation of energy and momentum to show that E1=((M^2+m1^2-m2^2)c^2)/(2M)

Homework Equations

E=√((p^2)(c^2)+(m^2)(c^4))=γmc^2
p=γmv

The Attempt at a Solution

Energy is conserved, so E0=E1+E2
The mass is initially at rest so E0=M(c^2)
E2=√((p2^2)(c^2)+(m2^2)(c^4))

So, I need to get
M(c^2)-√((p2^2)(c^2)+(m2^2)(c^4))
to look like
((M^2+m1^2-m2^2)c^2)/(2M)

 

[vela]

You need to use conservation of momentum as well.