Which statement$($s$)$ about a normal shockwave is $($are$)$ not correct $($check all that apply$)?$

$u_1{u}_2=a^{**2}$

Flow across a normal shockwave is isentropic$.$

$\frac{p_1}{p_2}=1+\frac{2}{+1}(M_1^2-1)$

$M_1{M}_2=1$