Shades of the "do winglets develop thrust" debate? But I guess what we're saying here is that a massive proportion of the compression actually took place ahead of the face of the first compressor, within the external shock system around the shock cone.
I guess its the three dimensional equivalent of the area velocity relation in quasi one dimensional flow:
dA/A=(M^2-1)*(du/u)
Integrate the pressure over the geometry of a convergent divergent intake and you should obtain the result.
Aerodynamics....full of surprises!
