1. The problem statement, all variables and given/known data Consider a light signal propagating in some arbitrary direction, with vx [itex]\neq[/itex] 0 vy [itex]\neq[/itex] 0 vz [itex]\neq[/itex] 0 and vx2 + vy2 + vz2 = c2 Use the Lorentz transformation equations for the components of velocity to show that v'x2 + v'y2 + v'z2 = c2 2. Relevant equations Combination of Velocities v'x = (vx - V)/(1-vxV/c2) v'y = (vy√1-V2/c2))/(1-vxV/c2) v'z = (vz√1-V2/c2))/(1-vxV/c2) 3. The attempt at a solution I know this is just a simple algebra proof but for some reason I'm getting stuck on it. Maybe I'm using the wrong equations? I would really appreciate being shown where to start for this proof. Thanks in advance for the help and/or time.