dc dy M ( D) Substitute Eq. D) into Eq (A) (M2-1 l) M 1> Supersonic region dc>0, then df>0 M<1 Subsonic region If dc>o, then df<ov dv c dc M = 2 (D) Substitute Eq.(D) into Eq.(A) c dc M f df ( 1) 2 = − M  1 If dc  0, then df  0 Supersonic region M 1 Subsonic region If dc  0, then df  0