9. In rectangle $A B C D$, $A D=a, A B=b(b>a)$, fold the rectangle so that point $A$ coincides with point $C$, resulting in the crease $M N$. Then open the half-plane $D A M N$ so that it forms a $57^{\circ}$ dihedral angle with the half-plane $M N C B$. The angle formed between line $A C$ and line $M N$ is $\qquad$
9. $90^{\circ}$

Connect $A C$ to intersect $M N$ at point $O$. From the crease, we know that $A O$ coincides with $C O$. Therefore, $A O \perp M N, M N \perp$ plane $A O C$.
So $M N \perp A C$.