1. The problem statement, all variables and given/known data In a two-particle collision the square of the invariant mass is. M is total mass of the system (M2 is the square of the total mass) m is the mass of each particles E is the energy of each particles p is momentum vector of eache particles 2. Relevant equations M2 = (E1+E2)2 - (p1+p2)2 = m12+m22+2(E1E2 - p1 ⋅ p2) now know the each energy E = mc2, here set the c =1 (Speed of Light), 3. The attempt at a solution But how the momentum have dot product in this equation ? How the dot product p1 ⋅ p2 can derive from (p1+p2)2 ?