Equation (8.80) does not hold for an oblique shock wave, and hence the column in App. B labeled P0,2/P1 cannot be used, in conjunction with the normal component of the upstream Mach number, to obtain the total pressure behind an oblique shock wave. On the other hand, the column labeled p0,2 / P0,1 can be used for an oblique shock wave, using Mn,1. Explain why all this is so.