about relativistic addition of velocitiesfirst, want to state that, no object can travel faster than the speed of light in ANY reference frame. second,I think in your case, A is travelling in the speed of 0.9C while B is at 0.9C but opposite direction. Well, A in the reference frame of B, its velocity is NOT 0.95C. You can disprove this from u=(v+u')/(1+vu'/c^2). My result from calculation is 0.994C. third, the relative velocity is the velocity of A with respect to B. There is no point to say, the relative velocity of A and B in the frame M, is 1.8C. Waiting for your enlightenment.