Hi,
I know this is a very old post, but the answer x'(2)-x'(1)=Lsqrt[(1-b)/(1+b)] is obviously wrong, as indicated by the correct solution: [..]prove that in S' the distance between them is L*(c+v)^0.5/(c-v)^0.5[..]
To obtain that, you have to pick a rest frame at first. If we take S' as a...