1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution E2 - p2c2 = E02 I know that this is true. But how do i relate p1 to p1'? and same for energy as well. I expanded the LHS= E0,12 + E0,22 + 2E1E2 - 2(p1c)(p2c) for the RHS = E0,12 + E0,22 + 2E'1E'2 - 2(p'1c)(p'2c) Obviously the rest mass energies equate on both LHS and RHS but how do i show that the 2E1E2 - 2(p1c)(p2c) terms are the same?