Solution
= Solution
Insert $h=γp/[ρ(γ-1)]$, eliminate $p_2$ and the downstream normal speed using mass conservation, and solve for the density ratio. This gives
$$
\boxed{{ρ_1\overρ_2}={2c_1^2+(γ-1)U^2\over(γ+1)U^2},\qquad c_1^2={γp_1\overρ_1}.}
$$