Equation (8.80) does not hold for an oblique shock wave, and hence the column in App. B labeled $p_{0.2} / p_{\text {, }}$ cannot he 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 $p_{4,2} / p_{n_2,}$ can be used for an oblique shock wave, using $M_{\text {..1. }}$. Explain why all this is so.